Maybe you're becoming an introvert. It's not a bad thing, it just means you like alone time. I love alone time, it gives me a chance to relax, read & write, listen to & make music, and pretty much just exercise my brain and intellect. I often enjoy it more than spending time with friends.
Man, I'm the same. People often perceive being with friends or hanging out with them would be a good time, but not for me. Me, a good time is when I'm alone, I'm a very relaxed and chill person. Don't get me wrong, I do enjoy going to the park often to play some basketball or lift weights with my brother and friends, but I'd still much rather stay at home, at peace. I like a very calm, relaxed environment. It helps me relax and not worry too much of a lot of things. I mainly do this because I have a very very short temper, and to prevent loosing my cool, I stay in a peaceful environment and relax. Yuh know, watch movies, play games, sit/lye down, listen to music, all the good stuff.
I think all teenagers go through ups and downs socially and spend some time wanting to take a break from the high emotions of their time in life. It's time to learn who you and are who you want to be. Time to reflect on your mistakes and your priorities. Teens years are the most social and the most emotional time of life. Taking time alone is good as long as you don't completely check out emotionally. You are just figuring out life and friendships and relationships with other girls and other guys and your family can be challenging when you have a lot of stuff going on in your own head. It will pass because you will grow and become your unique self and figure out how you fit in with the rest of the world.