Are you waiting longer between washes or have you just stared wearing your hair curly? Both will result in loose hairs staying in your hair, as they don't just fall out during the day like with people with straight hair. Then, when you wash, all the hairs come out at once.
It is normal to lose 50-150 hairs a day (can vary)...I know every time I wash and detangle (almost daily) I havea huge hunk of hair in the drain.
I did go through a time when I lost more hair than normal and was starting to see bald spots but there was a medical reason. Once I got past that, my hair started to grow back in. But I still lose a lot in the shower.
If you can't figure out why this is happening I would go see a doctor about it.