My jaw tightens up after i start chewing food. what can this be a sign of?

Possibly TMJ or ear: It is probably related to a bite issue. This can cause an imbalance in the muscles used to chew. Chewing should be a subconscious act. If you can only chew on one side or have to think about your chewing you are prone to this type of problem. You should see your dentist or an orthodontist to eval your bite and test which muscles are involved. In the meantime try eating soft foods and applying ice.

Pain When Chewing: Assuming your teeth are in good shape (and they probably are or they would hurt other times too) and considering your age, my best guess is that your wisdom teeth are trying to come in and you do not have room for them. The upper teeth could be pushing on your jaw bone and the lower teeth on the other teeth and either way causing pain. Either way, you need to see your dentist for an x-ray.

TMJ: You can be developing "tmj" or temporomandibular joint problems. You should see your dentist for a nightguard.

Tmj: See your dentist. Sounds like the muscles involved in mastication are spasming. An exam is necessary to diagnose this problem.

Yes: It is possible you may have some damage in your TMJ (temporomandibular joint which acts like a sliding hinge, connecting your jawbone to your skull) a consult with a dentist who treats TMJ or and oral surgeon is in order.