Boy I wish you knew what caused such a reaction.
That would be something you wouldn't want to happen again.
Did they say it would be ok for you to take some Benadyl?
IF so take that as directed on the label to help the rest of the swelling to go down.
It can make you sleepy though.
You could also apply an ice pack to help with the swelling.
It could take a couple of days to get back to normal depending on the severity of the reaction.
Of coarse if your swelling increased get to the doctor asap.