@nbkod7b Lori found a great link about trismus, and I wanted to add a bit from my own experience with physical therapy. I do tend to get TMJ type issues and have head, jaw and neck alignment issues with having a spine issue and thoracic outlet syndrome. Both of those affect my neck with tightness, and my physical therapist has worked on my pterygoid muscles which do get too tight with clenching my jaw. The pterygoids are connected right behind the lower molars and you can feel them with your tongue. Often dentists inject anaesthetic in them, and the needle can also nick a muscle fiber and cause inflammation starting the locking of the jaw. (that has happened to me) If you can get a finger inside your mouth, you would be able to massage and press of the pterygoids which is what my PT recommends for me to do.

The other muscles for closing the jaw are the masseters and if you put your hands on your face just in front of the jaw you can feel them. If you clench, you will feel them contract. You can press on them with your palms and just rest the weight of your head on your hands to help release them. My PT has me do this too, and she is an expert on myofascial release which has helped me a lot. I would also echo what Lori said about trying some physical therapy and myofascial release.

Here is a discussion on MFR where you can learn more. https://connect.mayoclinic.org/discussion/myofascial-release-therapy-mfr-for-treating-compression-and-pain/

You can search for a MFR therapist at this link. http://mfrtherapists.com/

Have you heard of myofascial release before?