I used to have very thick hair, now my hair is thinning and I probably have lost half of what I had but I'm only 14! Well the doctor said nothing but I've been taking chew-able vitamins and iron supplements, as well I have just lost 60 pounds, could the diet have affected my hair growth? How can I re-grow my hair? I don't straighten, curl it or ever use an ironing iron for my hair.
Shampoo can't be the problem right? Since I keep changing every week or day.

Its okay, I started noticing a lot of my hair was falling out. It's a stage most likely and you can read more about it on the enternet. There are shampoos and condioners out there to make hair thick you might want to try those. Otherwise give yopur hair time and stay away from hair dyes. Goodluck :)