What. Would be the cause of real bad jaw pain?

Many. Trauma, tooth decay, infected tooth, periodontal disease, benign tumor, malignant tumor, muscle pain, TMJ (jaw joint) problems, ear infection(referred to jaw), See oral surgeon for advice.
Probably. Tmj but could also be referred from somewhere else, even your heart so do not ignore it especially if there are any other symptoms . Sometimes women don't have the classic heart pain or other symptoms of a heart attack so if this is new for you take an Aspirin and go to the er and be on the safe side.
Jaw pain. Causes will vary by location and and any precipitating events. If pains severe, you should see local doctor to evaluate you and review your history to determine what could be cauisng it.
Several things. You have an upper jaw (maxilla) and lower jaw (mandible) composed of different tissues, muscles, bones, ligaments, teeth, and glands. Jaw pain can be from any of those structures as well as how they function. You need to have a thorough clinical examination with the appropriate studies to diagnose where your pain is and what is causing it.