I'm a 14 year old girl. I used to have really thick hair but lately ALOT has been falling out. What do I do?? And why is this happening?

Hair loss?? Your hair loss is called alopecia. You should see a dermatologist who will do a biopsy of your scalp and see under a microscope what is happening. He/She may suggest injection of a steroid into your scalp. Good web sites for more I sites are: www.mayoclinic.com and www.johnshopkins.com and search term alopecia. Good luck!