I spent some time being a ginna pig for a dental student. *That* was a learning experience. The current dental student classes are being taught that 'air pockets' under fillings don't exist. Their reason for pain after a flight is 'the change in altitude'. I confirmed the teachings with the professors. Then, I let my student know the profs were full of it, having once nearly gone into shock from the pain after a flight due to an 'air pocket' under a loose filling. That was a fun root canal.
A few months previous, I'd had problems after a flight that a dentist thought was wisdom teeth problems. It wasn't -- the wisdom teeth were causing pressure on the loose filling up front, in the tooth that later needed the root canal. The wisdom teeth removal only temporarily solved that problem.
TMJ can be a cause of jaw pain, same with:
Sinus problems
Allergies (even to a food caught between the teeth!)
the previously mentioned Bruxism, although the cracked teeth can have deep cavities
cavities under fillings
loose fillings (the ones put in as kids do corrode and wear out)
David Clamps or other tight headsets causing pressure on the ear or jaw
Eyeglass stems that don't fit right
Ear problems, infections, water in the ear, and so on
Clenching one's teeth from stress (at night, ever dream about your job, and the associated desire to choke the living daylights out of the [pick one: Captain/FO/FA/PAX/Dispatcher/FE/Controller/CFI/MGMT/Union guys/idiot in the rice rocket that just cut you off/weather dude/scheduler/kids/spouse/girlfriend/guyfriend/student/bill collector/etc]
Muscle spasms in the neck (working out too hard, the oh BLEEP! traffic call, too many hours on flightinfo)
Broken jaw (ever fall out of bed or tick off the girlfriend?)
A dislocating jaw (subluxation) - can be really fun if it stays out for a while then snaps back in
Start with the DDS. If no fix, try the AME (call for advice first), then an ENT (ear-nose-throat).
For all those that hate going to the dentist, keep in mind that some of them are pilots, and those pilots have to see a flight instructor every two years at a minimum. So what do I tell my DDS/flight student before he starts doing anything? "Remember, paybacks are a b*tch."