Why do i feel sad for no appearant reason?

My problem is that I'm always feeling sad& lonely even though I am surrounded by a few friends and a loving boyfriend. I a few friends and a loving boyfriend. I know many people ask why, but It's the kind of feeling I can't help but feel like that. I try venting it out and ignoring it by trying to have fun with friends, but the feeling always lurks in my mind and eventually finds itself back in. I've had this feeling for the longest time but I've never put much thought into it until recently. Each day I come back home from school (which is around 4:15pm) I tend to eat my meal and immediately go to sleep until the next morning to attend school at 7:00am. If I don't fall asleep I lay awake in bed until I do. I always tell myself to get up and draw which used to be one of my favorite hobbies to do all the time, but for almost a year I have found no interest or motivation in it. Even as I am writting this I am laying down in bed waiting to fall asleep. Each day this repeats itself, even on the weekends as I am not allowed to go outside. I feel it would be helpfull if someone could suggest what might possibly be wrong with me.