One, clean it up with mouthwash. Then if it's really swollen use ice to bring it down. Toothpaste can usually tone it down, but most important just leave it alone after doing all of this (I know it's hard) the longer it stays untouched the better (specially overnight) by tomorrow it might still be there, just not too noticeable like today (and yes, get some abreva for the future)
Press a cold ice cube against it. It has to be in the early stages though. I swear mine don't even appear if I do this ASAP!
Abreeva- from the drug store
I know that toothpaste zaps zits. You could try that?