How do I even have hair left????

I can't wear my hair down anymore because it falls out all over the place and drives me nuts. And I mean falling out in CHUNKS! I ran my fingers through my hair and caught a ton of loose hairs and my husband was horrified by how much is just falling out of my head. It's starting to look really thin in the middle of the back of my head. I mean is this normal? I knew it'd fall out but not this bad.