Crap. I necroed it… it said august lol, didnt show the year.
No problem. 
Just for the record, what would your response have been?
Whether zombie or regular human, that somebody would have this symptom sounds perfectly realistic to me. I get kidney stones, and when they’re high up in the ureters (and therefore in the lower mid back not far from the bottom ribs) they often make my balls ache. I’m not talking about the intense flank pain of renal colic. I think it’s just some irritation. And it gets misrepresented as coming from the testicles.
Not that I think you have stones every time you eat too much pizza. I just accept that testicles can be the target of misdirected pain.
Here’s a shot in the dark theory. Since your gonads have nothing to do with your digestive system, it can’t be the actual food itself.
When you eat pizza, you probably scarf a lot, which means your stomach is very full, which means it exerts pressure on other organs. If you eat a lot in general, do you ever exhibit the same symptoms?
It did though. But you have to recognize that the teensy tiny apostrophe to the left of the two digits is this forum software’s cack-headed way of expressing a two digit abbreviated year (like we never learned anything from Y2K).
Still, I find myself wondering the outcome of @Larry_Mudd’s mysterious medical misfortune.
I used to have the same problem, before I took to putting on pants before I put the hot pizza in my lap.
Pretty sure this is the still the most embarrassing question I’ve ever asked on the internet.
Also, hey folks! How ya been?
I just now noticed the OP asks why does “overindulging in pizza make your testicles ache”, not why it makes their own testicles ache. How presumptuous!
I’m intrigued… do your testicles still ache when you eat pizza?
Since your gonads have nothing to do with your digestive system, it can’t be the actual food itself.
I don’t think that premise is entirely correct - eg; there are plenty of foods that you can eat which manifest unusual symptoms outside the digestive system, because ultimately some of the constituents of your food will find their way into your blood and be distributed throughout the body. Allergens, vasoconstrictive elements, things like capsicum can all manifest symptoms throughout the body.
Turns out for me it’s just cheese. Doesn’t have to be pizza, but twenty years ago pizza was the only way I ever found myself eating entirely too much cheese. Once it hits the ol’ small intestine, my scrotum would go into Iron Fist mode until everything was cleared out. This has not been an issue since I gained the wisdom and culinary ability to avoid eating entirely too much cheese.
How is it possible I’m talking about this again?
Fighting ignorance is what we do here, so thank you.
Welcome back Larry. I hope you stick around.
Wow! I stand corrected! 
Um … that would be my fault. LOL
I noticed that this thread is close to 20 years old, but still interested me since I have a similar (but not exact) affliction. I generally top my pizza with garlic powder and red pepper. If the red pepper flakes are full of seeds, and if I haven’t eaten spicy food in a while, I get this dull annoying pain in that general vicinity. Not the testicles, exactly, but more behind them, above the taint area. I mentioned this to the doc once years ago, and he said it’s likely just a kind of irritation that might be causing minor temporary inflammation. Not sure if the OP puts red pepper on his pizza, but that might be another source.
I Googled this issue, and it appears you are on track with that. It also mentions food allergies.
Testicle Pain after Eating Spicy Foods
I’ve been having stuff like this happen for over a year now (started when I was 19; I’m 20 now). I been to doctor A LOT and to the ER a few times. I’ve had many ultrasounds, x-rays, and a cat scan and a colonoscopy. Bottom line for me: nothing showed up and pain pills weren’t a help. Eventually my main doctor did blood work on me and I found out I was allergic too a lot of foods. It was a shocker because I had been eating things like wheat, tomatoes, and other stuff since I was a kid but I suppose my body had a minor a intolerance to these food till now (some allergies can develop over time). Anyhow I omitted about fifteen different types of food items and some spicy stuff (I actually feel bad after eating those new tapatío frito chips) and I feel a lot better.
Ya know, usually I am the guy who takes these sort of things at face value and needs to have the joke explained to me.
This time I was sure I was savvy to the euphemism to extended periods of sexual activity. Well done Orville!
But nope, not a euphemism. 
Anyway, Larry_Mudd, glad you got it figured out!
I was going to guess too much salt. I find that if I eat too much salt, I get issues with my urinary system for the next day or so, and that’s pretty close to the gonads.
Welcome back. And it was an interesting thread, even if embarrassing.
Thanks! What’s interesting is that my body adjusts. So when I eat it regularly (as I’ve been recently), it’s never an issue. Also happens with other foods that contain many hot pepper seeds. I remember some particularly spicy jalapeño poppers and with some of the ramen soups I buy from the Japanese store.
Thank you for the answer to the puzzle. I did wonder when the zombie thread turned up, but I never expected you to come back.