Is it weird to not like socializing?

I’m 21 – the greatest time to be a social butterfly, right? Not for me. It’s not that I don’t like people, I have friends and everything, but they like hanging out 3–4 times a week and I’m the opposite. On occasion I like being social, but only like once every 2 or 3 weeks. I have fun when I do socialize, but I am completely content staying at my house. So I guess it’s more or less “indifference”. There’s a lot of times when my friends call me to invite me to go out to eat or go see a movie and I just don’t want to or don’t feel like it and I can’t figure out why. Is this normal?