What Causes Thinning Of Hair?

Hi, I am a 24 year old female and I have noticed my hair has been significantly thinning. There is no history of hair loss on any side of my family, especially among women. I thought it was antidepredssants I was on, but it was not. I got my thyroids checked, and they were fine. I was wondering if maybe the fact that I drink alot of diet coke might have something to do with it, or what else?