You made a mistake, everyone does.
I think you should tell him. Let him know it's a mistake you feel really terrible about and just ask for his forgiveness. If he says he forgives you and then tries to "get back" at you, dump him! You don't need to deal with the guilt of having a drunken night and now your boyfriend thinking you need to be punished.
It's not worth the heartache.
EDIT: sorry didn't see your question about morning after pill. When I took it I was sick as a dog for the next 3 days. Vomiting and terrible cramps. Also when I finally got my period it was extremely crampy, more than normal. I am also sensitive to medicine so I have unusual reactions, you may experience just fatigue or nausea