Hair Disaster! Please Help!?

Okay, my hair has always been thick and luscious. But a few weeks ago, I started putting my hair back in tight ponytails... I would flip my hair and brush it, but now it seems like its getting thinner! Now I'm getting really paranoid because ever since I've put it back it hasn't been so thick and luscious, I feel like I'm losing hair! Please give me some ways to thicken my hair and revive it! I'm only in my young teenage years and I want to do something before its too late. *P.S. I use Tresemme, could this shampoo and conditioner be weighing down my hair?*