get over depression

my sister was depression for long time, and she is manic-depression. (sorry im not engligsh speaker, so im not sure if its correct.) and now Im so depressed. I really dont know if this is depression, but I cant think in a good way latelly. I started going bikram yoga to get over with stress, and it works, but only in the class and after the class, about 2,3hours after the class. I was about to sleep soon today, but i couldnt get sleep because i felt the same feeling when I was 5,6years old at home alone. I live by myself for 1 year, and ive never felt that way since i moved to here. I dont feel any depression right now maybe because i dont have to think, also when Im with my friends, and when im at school. What I can say right now is i dont wanna be alone.