My boyfriend got upset with me and I don't know why, what should I do?
Can you not think of any reason that he would be upset with you?
I've only had one drink, so I really don't get it.
I thin he's being a bit overprotective if he's angry at you for one drink.  He needs to realize your an adult who is capable of taking care of yourself!
Maybe. But it's really not a big deal.
You're right, and you should tell him that you drinking a little bit isn't a big deal. It's good to stand up for yourself in a relationship so that your partner doesn't get overly controlling.