Is my horse in pain while using her bit?

hi, my mare has started severly tossing her head and backing up, and a few times even rearing, when any pull other than mild is applied to her bit. her bit is the right size and we can see no other apparent reason that she should be doing this. I heard on a television show that a horses back teeth can be in pain when using a bit. am i hurting my horse? how can i make her more comfortable while riding. or can this be fixed? Please, help me help my mare. thank you..natalie and emma.

My horse was also having problems similar to your horse, I had the vet float her teeth and it helped a lot.

What type of bit are you using? How strong is it on her mouth? And is it the correct type of bit for your ability, her ability, her work and her temparament?
   
It sounds like she is in discomfort. Although any horse will react when you pull it in the mouth, but the severity of the reaction depends on the pain it experiences.
   
Get her teeth checked right away and speak to a BHS qualified instructor about her bit.
   
If she is quiet, I would go back to a rubber happy mouth snaffle and see if that helps with this problem. She may simply be over bitted.
   
But seek professional advice as fast as you can, otherwise this could cuase a longer term problem if it continues.