How to stop teenage hair loss?

Very recently, my hair has been falling out vigorously. When I put my fingers through my hair, at least 3-4 hairs come out, and that's every time I do it! I am a 16 year old girl and I have a good diet. I am not really stressed out or anything, so I don't know why my hair keeps falling out. I already have thin hair and don't want to lose more! Is there anything I can do to stop this? Any vitamins I can take?