Hello and welcome to the Anxiety Community!
Alcohol absolutely positively triggers anxiety in a LOT of people, especially ones who are already prone to it. Usually, a person will have a spike in anxiety the days following indulgence with alcohol, and obviously, the more you drink, the worse the anxiety seems to be.
If you search the forum, you will find a lot of topics about this. There is one long and old thread though, that I think will give you a lot of insight into this issue. I'll find it for you and post a link to it. Read through the replies...I think you'll be amazed at just how comon this is.
You're 21 and drinking is probably a pretty common part of your social life. The best thing for anxiety sufferers is to stay away from alcohol all together, but realistically, I know people (especially young ones like yourself) aren't going to do that. Therefore, my advice to you would be to seriously ONLY drink in moderation and occasionally. Try not to drink to an extreme level of intoxication...that will make the rebound anxiety terrible.
If you are responsible about your drinking and don't "overdo" it, hopefully this shouldn't be too much of an issue for you. However, conversely, if you are consistently noticing anxiety after a night out, you may seriously want to consider curtailing the drinking in a big way.
I'm going to find that thread for you to read. If you have something to add, post it on THIS thread...not the old one. The older threads don't get as much attention, as a lot of members tend to skip over them.
Best of luck...and HAPPY BIRTHDAY! :0)