That's just how us introverts are and I hope that no one takes it the wrong way. It's not that we don't like you or other people, it's just that our personalities can only handle so much when it comes to being social. After hanging out with a friend (yes, it could be just one friend), I need some alone time to recharge. After spending time with family, I will often go off on my own, usually to my bedroom, to just do something on my own like go on my laptop or read a book. Sometimes I can't wait until I'm done spending time with family and friends to spend time alone and I will spend an extra few minutes in the bathroom during a bathroom break just being, well, alone.
Maybe this sounds strange to some people but ask an introvert and they will most likely tell you that they too need some alone time. Being social and being around people can be really draining. It feels like I get zapped of energy or that I get stressed out from all of the interactions. Being alone gives me time to just breathe and think all by myself. When I'm alone, even in just a bathroom, it feels like I finally have some space. I finally have some room to breathe and relax my mind. I don't have to focus on what other people are saying or on what is happening around me. I don't have to contribute anything to the conversation or activity. I don't have to think about specific things. I don't have to do anything, really.
So what does it feel like after having some introvert alone time? I personally feel recharged. I feel relaxed and refreshed. It's like how you feel after taking a nice shower or waking up from a great sleep. You feel happy and full of energy. It's amazing! I think of my alone time as a form of meditation, in a way. It relaxes me and energizes me, much like deep, slow breathing does during meditation. If I need to go back to a social situation after my alone time, then that's fine because I feel ready to go back. If I don't need to go back to a social situation, then that's even better because it means that I can have a longer session of introvert alone time.
We all need to be alone sometimes. Introverts just need it more than others.