Hello,
It can be due to seborrheic dermatitis, fungal infection or telogen effluvium. It is nonscarring alopecia characterized by diffuse hair shedding, often with an acute onset. There is a chronic form also which persists for a longer duration and starts with insidious onset. Chronic telogen effluvium is hair shedding lasting longer than 6 months. Treatment consists of medications like Minoxidil, surgical approach(hair transplantation) and improvement in diet.
My sincere advice is to consult a trichologist. It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
I've never had that happen to me but I'm 16 and because I dyed my hair so much it is thinner now than it used to be. I've switched to Mane N' Tail shampoo last week and my hair looks healthier and feels better because of it. It doesn't look much thicker but it does feel and look better, and as long as it keeps your hair strong then the damaged hair will be able to grow out so it should get thicker and healthier after a while.
You can get Mane N' Tale from Sally Beauty supply for around $6.50 a bottle (in Canada anyway, I don't know the pricing in America), and I've also heard you can get it from Wal-Mart. In the states Target or K-Mart may also have it.
Hope this helps! :)