Mysterious hair loss?

For about a month now, my hair has been falling out drastically. It started falling out when school started. I seriously don't know why, and it's starting to really scare me. I lose about 100 or more strands of hair shower and about 50-100 more throughout the day. I don't have any signs of illness or unhealthiness. I've never had this problem ever throughout my life. And to add to the problem, my hair is already thin. So I'm more than petrified to start balding......and that would really suck 'cause I'm a girl and well, girl's don't usually have bald heads. What's wrong with me? :(. Oh, and maybe I should add that my period is 2 months late.