Tony had an allergy to something -- I think it was a food allergy maybe -- but the other Avengers didn't really believe him. So he kept sucking it up and doing whatever it was they were doing just to be friendly, even though he knew he'd have a reaction. I think it was a 5 things fic, or maybe a 5+1.
If you find this for me, I would be delighted. ^_^ It's really bugging me right now for some reason.