Old British music hall song:
I can’t do my bally bottom button up,
Can’t do my bally bottom button up.
It’s so tight, serves me right,
Must have eaten too much grub last night!
Since the Op sounds like a woman one other possibility is the cut of her pants. Women’s clothing manufacturers assume we all have waspish figures and make the waistline of pants a lot smaller than the hips. If she’s built like me she buys pants that fit her hips but cinch her waist too tightly. You almost have to buy men’s pants to be comfortable.
What you do in private, or in the sole company of friends and family (as long as they don’t mind), is none of my business. But someone unbuttoning in public (e.g., restaurant of party) is gross to me. It’s like a person at the next table using a toothpick. It doesn’t actually affect me, but it seems low class and squicky.
What you’re experiencing isn’t normal for me, and I agree that you should look into food sensitivities and/or stretchy pants.