Name changed but fear I'm totally outing myself nonethless...
After collecting dd1 from her pre-school classroom, we're (dd1 and me carrying baby dd2) walking back through the enclosed communal play area to the heavy glass door which leads into reception and the front entrance. It is busy as always. I reach the door which is being held open by parent A. The parent and child in front of me walk through it to which parent A says "I was holding it open for the man with the pushchair" and gestures to parent B who has one dc in the pram and is obviously waiting to come through to fetch another dc.
I've seen parent B regularly on the pre-school run and he has always been pleasant. I don't know his name, his kids, don't strike up conversation etc just share the normal everyday niceties.
So when parent A says that about the door, I call dd1 back, smile and say "here you go" to parent B, indicating he can come through the door being held open.
To my astonishment he snapped "Just come on through for God's sake. There's no need. Just come through the door." Parent A walked through the door with me and mine behind, followed by other parents and kids. Parent B is still muttering - loudly - at this point as we all flow through the door and go past him. I apologised (no, i dont why either!) and said "I was only trying to help". He snapped back "well you weren't were you? There's one of me and 20-odd people behind you. I'll wait. You're just causing problems" etc etc and continued to mutter. I was cross at his attitude and was starting to feel embarrassed at being shouted at so I just turned and said "there's no need to be like that. I'm sorry that you're obviously having a bad day" and walked off. He was STILL muttering about me as he finally got through the internal door and walked off to his dc's classroom.
I resisted the urge to go after him (I can't stand mutterers in situatons like that - have the balls say it to my face not under your breath!) and seethed all the way home but then burst into tears when we got in.
I was only trying to be polite. Why am I letting him upset me - AIBU? And can I let the door close in his face the next time he's stood there? 
Sorry for the long post.