My jaw on the left side will radomly lock up and it becomes very painful and hard to eat why is this?

TMJ syndrome: You may have a disorder called temporal mandibular joint (TMJ) syndrome. It can be caused by clenching teeth at night, trauma, malalignment, abnormal jaw thrust when eating, lack of an overbite, or other causes. Treatment may involve wearing a mouth guard, dental work, analgesics, or biofeedback. A visit to your primary care physician or dentist can properly give you a diagnosis and treatment.

Joint problems: It is possible that the temporomandibular joint -(tmj) joint has a problem. The capsule in the joint may be dysfunctional or the you may be dislocating your joint from time to time. This will make it both painful and hard to chew. You would need to see a specialist who specializes in TMJ care to evaluate this joint. This is classically an oral surgeon.