What's wrong with me?

I have had stages through my life where I just feel very down and lonely. I usually dont smile much and am very distant and empty. Despite feling lonely, I ignore my phone and dont open the door, this doesn't really make sense because I feel lonely yet I dont want to talk to anyone. I dont know why I do this. It happens maybe every 3-4 months and last about 3 weeks then im back to normal and chatting to people. Why do i shut myself off like that ?