Hi @Shakalker
That's one thing i have not had being on letrozole but had something like that years ago after having our second child, went to the dentist, they found nothing but suggested it could have been an infection in my sinus' which can refer pain to your teeth area. Could also be an ear infection because that can translate pain to your jawline, I've had that before. One last suggestion, if my shoulders are really playing up pain wise this can lead to pain in the jawline too. Good excuse for a massage!
Good luck with it all and as @kmakm said pilates is great, whether it's mat or reformer. Try and find a studio that concentrates on technique so you learn to do the exercises properly, learning which muscle groups to switch on and which to switch off ( that's the hard part) and matching your breathing to the exercise. I started off in a studio run by my osteo which was great as it very much centred on doing it right rather than doing it fast. Cathxx