You can have two different things wrong, one of them your jaw itself and the other your chin. Here are two exercises you can do to get rid of that pain.
While sitting place your hands on top of your head so your thumbs are behind your ears. Now press your thumbs in where they are and you should be just below the skull and at the back of your jaw. Position your thumbs so they are pushing on the back of the jaw bone but also next to the skull when you do so. Press in pretty hard and hold. then relax, take a deep breath and exhale and don't tense up any part of your body. After 30 seconds, slowly raise your head until your neck is fully extended. Then open your mouth all the way and hold it like that. Now release the pressure but hold your head and mouth like that for one minute more.
then do this one for anything in your mouth:
Under your chin: Place your thumbs up under your chin near the front. Feel the muscle between them for as tight as it can be. Press on it and hold. Relax, take a deep breath and exhale and don’t tense up any part of your body. When the muscle starts to release, slowly look upward until your neck is fully extended. Then release the pressure but hold your head there for one minute longer.