I swear people only offer me seats because they think I'm pregnant. I'm gonna start calling them out for it.
Why do you want to call people out for that?
I'm pretty sure they're just doing it as an underhanded way of telling me I'm fat.
If you really think that's what they're suggesting then I totally get why you're upset and you should totally stand up for yourself. But what about the ones who are actually just trying to be nice and are making a mistake. I think it would be better if you just quietly told them that you aren't pregnant and let them be embarrassed like that.
I would, if it weren't for their judging eyes and comments about it later.
I think maybe the best solution is to ask them, are they calling you fat. If they are, you can stand up for yourself, if not give them a chance to apologize for mistaking you for being pregnant.