It could be several things but do have her teeth checked. I have one filly that doesn't have any noticeable teeth problems. I had the vet check her and she has definitely got some hooks that need taken care of before I stick a bit in her mouth again. I can stick my finger in her mouth and not feel a thing but the vet can find them right off
I'd make a lousy equine dentist.
Three years old is time to start getting them floated.