Do i have social anxiety?

i just got back from my safe grad.. and on the bus people were being really loud and obnoxious, and it really upset me.. i got really depressed and didn't want to be there.. i feel like i'm fine with a couple of people around but i can't tolerate large groups/parties. sometimes i think about death. also i usually need a couple of days to stay at home and not see anybody for a couple of days afterwards until i start feeling normal again. when i looked around everyone else was happy and having fun but i really can't seem to have fun in social gatherings with a lot of people.. what's wrong with me?