The relationship between TMD (temporomandibular disorder) and braces is ill-understood, even by the professionals. Sometimes having braces can make TMD worse, and sometimes it can make it better, If you already suffer from TMD, yes, you absolutely must make sure that this is something you discuss with the
orthodontist before starting treatment - and realise that some
orthodontists will not be willing to start treatment if you do suffer from TMD.
Discomfort in the teeth and gums is pretty much normal when undergoing orthodontic treatment, but pain in the jaw muscles and especially in the TMJ (temporomandibular joint) bears closer scrutiny. It's certainly possible that as the bite is shifted during treatment you can experience periods of time where the jaw joints and jaw muscles become achy due to the forces they are under. As the bite improves, this should be alleviated. However, if the pain is more intense, or if it persists, it's certainly worth discussing with your orthodontist.
Julie, it's terrible that your orthodontist made light of such debilitating pain. I wholeheartedly agree with your advice that if someone should find themselves in a similar situation, and their
ortho similarly dismisses their concern, they should most strongly consider seeking a second opinion.