Why am I crying, I don't know why I'm sad.?

I used to have this problem, and my sister is going through it now. Go to the doctor. It may be because of hormone imbalance. Usually that is the problem. Especially since you are 13, your hormones may be WAY off. Check online for any side affects of off balance hormones. Try to have some relaxing quiet time too. Sit in a bubble bath with bath salts and just try to rest. Lay in bed and listen to your ipod, or when you are calm go talk to your mom without showing emotion. Come up with compromises.