Yes, I agree with Monisajda, see your doctor, it could be something causing this that requires attention.
Aside from that, stress can cause it, pregnancy (hormones) and strangely enough, if you do not get your hair cut/trimmed often. Sound weird? This was actually my problem, I didn't have a lot of money so getting a hair cut was not at the top of my list. I asked a salon owner that's been in the hair business for 30 years, he said it was because I was going too long between hair cuts.
Apparently, you should not go longer than 6 weeks without a trim, 8 at the absolute most. You don't need to have much taken off, 1/8th of an inch if you want.
He also recommended getting a good brush that would stimulate my scalp, causing the blood to circulate near my scalp's surface and brush 100 strokes a day, which stimulates hair growth (according to him).
I can't scientifically verify if any of what he said is true, but I can tell you that I have been diligent about getting my hair trimmed regularly now and my hair has stopped falling out like crazy.
Hope this info helps :-)