It's not selfish to want to be by yourself: it's actually quite healthy! So if being a social butterfly is simply not high on your priority list right now, don't be ashamed of it. Rent one of your favorite movies and get comfy on the couch tonight. Order in some food and just relax. Social-hound friends may not understand this inward phase you're going through, but they will accept it: they'll have to! You need to be honest with your needs. People appreciate it.