I feel for you! Know where you are and how the oncologist are. Maybe check in with your ENT, oncology PA/head nurse and a physical therapist. Many of the oncologist live in a box and have tunnel vision. They do their thing you pay them and the contract is complete. < -- A little cold and dry, but this is how I was done. Acupuncture helped me with my lymphedema (my neck was as big as my head). Within 20 minutes it was half the size. They might be able to help with the jaw pain/feeling?! Careful with the pain meds... might be adding to the issue (side effect). You are in my prayers.

Hello @jazzyjoy and welcome to H&N group. All I got for you is to try to exercise your mouth and neck as much as possible. Alive or Tylenol seem to help although there are some stronger muscle spasm prescription meds however they usually have unwanted side effects. I also chewed gum slowly and softly which seemed to help.
I am so glad my spasms subsided but it really took years, not months. Maybe keep a log to see if things are improving with time. If no improvement or things get worse, talk with your oncologist or see what your GP has for a solution.
I wish the docs would tell us what a tough ride this is although then we might not hop on, which would be worse. You get through this you can get through anything.