Long term vegetarians will commonly say that eating meat for the first time after many years will cause vomiting or stomach aches or other issues. Does this happen with any other food? If I don’t eat peaches for a few years, I don’t have a hard time eating a peach.
If you eat a boatload of peaches after a prolonged no-fruit diet, you probably will feel pretty rotten.
We habituate to all kinds of food- oily food, spicy food, fiber heavy food, gas producing food, etc. Suddenly introducing a lot of beans or oil or whatever to your diet will have an effect on most people. Meat is no different.
FWIW, starting to eat meat was no problem for me after ten years as a vegetarian. But despite having no attachment to vegetarianism and being a pretty good omnivore, eating a large amount of beef still makes me sick every time.
I eat a lower-carb diet and try to get my carbs from fruit and veg. If I eat a large quantity of bread, I will have an upset stomach soon after.
You lost me right there. Anything they say I discount as nonsense.
Let’s eat some BBQ and drink Beer!
This has always been an interesting subject, or question, for me. So it has been with great interest that two friends, at different times in my life switched from vegetarian to being meat eaters. I asked them both if they noticed any difference and the answer was always “No”, that they hadn’t noticed any difference. YMMV.
My mother is not a vegetarian but after cutting beef out of her diet, trying it again made her feel bloated and generally uncomfortable, same with a coworker.
On the other end of things, my not-vegetarian husband regularly reports gas from beans and certain vegetables (like broccoli), to the point of rejecting split pea soup, but I (an ovo-lacto vegetarian) can eat them just fine.
I don’t know about me these days, as I haven’t confirmed any exposure to meat after the fact lately, but over a decade ago when my husband and I were living together, my father-in-law spiked my “vegetarian” pasta sauce with beef broth without saying anything. It was strongly flavored anyway and I didn’t suspect anything. I was feeling ill by the end of the visit and throwing up right after I got home. My now-husband got a triumphant call from his dad to the effect of “see, she can eat meat” to which the response was given that perhaps he could come over and take turns holding my hair back while I puked.
I also accidentally self-induced vomiting when I bought veggie egg rolls, ate them, threw up, then checked the label and saw chicken fat and broth buried in the ingredient list.
I’ll note that I frequently cook meat for my husband and others, I dissected animals in high school and college biology classes, I’ve helped hand-make sausage with natural casings. I don’t find meat innately revolting or anything.
Oh, and on a related note, I’ve never experienced the infamous “ring of fire” effect the next morning after eating really spicy-hot food. Digestion is weird.
I hope I am not getting to personal here, why can’t you digest meat? Is it just because you haven’t had it in a long time or is there some other digestion issue?
Quite a few years ago I was a vegetarian for about 8 years. I did experience what the OP described. The first time I ate fish I don’t recall any stomach problems. The first time I ate chicken I had some weird stomach issues. The first time I ate turkey, I remember very well. 
I had LOTS of stomach gurgling and mild nausea. I never got sick but it was very obvious my stomach didn’t necessarily like what was happening.
All of these symptoms quickly disappeared on the second and third eating.
J.
That’s my assumption. It’s apparently disagreeable to my stomach - though frankly, I’d always found beef and pork kind of made me feel all bloated. I assumed it was the amount of fat at the time, as they weren’t exactly lean cuts.
I’m apparently awesome at digesting beans, cruciferous veggies, and capsaicin, though! And no, I won’t give those up then return to them to test for digestive difficulty.
I gave up gluten a year ago because it was causing long-term digestive woes. Now, when I’m exposed, it seems to cause immediate, drastic effects that are way worse than the long-term problems I had been experiencing. Even if I wanted to start eating gluten again, I’m not sure I could at this point.
The question that we’re not going to solve (since it would involve using you as a lab rat and it wouldn’t be publishable anyway) is “is that because your body isn’t used to cereals, because it isn’t used to the specific cereal that bread is made of, or would it still happen if you were having bread every day (coeliac, gluten sensitivity)?”
I went back to eating meat after about 8 years as a vegetarian andn’t have any issues, athough I was somewhat cautious as I had heard the same thing.
if I go even a few days with a vegetable-poor food intake, and then have a decent sized green salad (e.g. one with actual greens and not just crunchy water) then it’ll run through me in about one hour.
which means (spoilered for gross TMI)
I will see leaves in the toilet bowl when I am done.
Yeah, I started low carb mostly because of a desire to lose weight, but I also immediately noticed the positive affects on my digestion. So it may well be that I have some mild condition.
I normally drink a lot of milk (easily a liter per day) and whenever I stop drinking it for more than a week or so, it then takes my intestines a day or so to get used to it again.
Acidic fruit can cause problems for me, but I’m not sure if it’s related to re-exposure after an absence. As Walton says above, milk will hit me when I’m not used to it.
I can vouch for Ferret Herder’s tolerance for meat on other people’s plates - she’s very kindly suggested I order meat if I’d prefer when we’ve gone out to lunch. She’s not a judgey vegetarian at all. 
I wonder if the vegetarians who have this problem are people who, if not their primary reason, included among the reasons they stopped eating meat in the first place the fact that their digestion isn’t best suited for meat. In other words, it’s easy to give up meat if meat makes one “feel all bloated”. So perhaps your illness with meat predates your vegetarianism, and only becomes even more dramatic after a long time without meat.
My ex husband wasn’t one of those - he was a squeamish vegetarian - the texture and *idea *of meat grossed him out. But when he eventually decided to eat meat again, there were no digestive issues, even though he was anticipating them. (And chose White Castles as his first meat containing meal, no less!)
I quit eating oatmeal some years back for no good reason. I started eating oatmeal again and it gave me a crazy stomach for the first few weeks.
how awesome of her to allow you to do so.