With any physical issues ruled out, if she was doing "just fine in that bit" sounds like user error. Not sure where you're at skill-wise, but have someone video your ride. Pay attention to your seat, balance, and your hands. Are your shoulders and arms locked, are you too busy with the reins? Or are you too passive letting her pull on you? A harsher bit isn't the answer.
Re: physical issues- remember TMJ problems can cause body pains and body pains can cause TMJ problems. You always have to look at the whole horse. Even if the teeth are fine, the lower jaw may be misaligned and painful.
Be sure and discuss all this with the horses' owner as well, leasing is not owning.