I find quite a bit information that says flexeril should not be taken when one has hyperthyroidism due to the possibility of causing heart attack, but I can't find no evidence that it triggers hypothyroidism, so I would say it did not trigger your hypothyroidism.
The best thing to do is talk to your doctor or pharmacist to find out about interaction between the flexeril and hypothyroidism.
TMJ is a possible symptom of hypothyroidism.