What could it be? Woke up with nikel size puss pocket on lower lip and pain down that side of my face (under jaw bone) all the way to start of neck just under jaw bone. Excruciating pain

Please . Please go to your dentist or physician today! an infection in the head and neck area is very serious! if you do not have a dentist or physician seek attention at an emergency room at your local hospital. If you cannot afford treatment, go to the nearest free clinic in your area.
Without . Without more information and a proper examination it is difficult at best and impossible at worst to diagnose your condition. Infections of the head and neck are potentially fatal and need to be treated promptly. It is all to easy for an infection in the head and neck area to get out of control and impinge on vital structures (such as your airways) which can lead to death or impinge on many non vital structures such as your eyes, ears, nose etc... Possibly leading to permanent damage of those structures. You really do need to be treated promptly. As you were advised in an earlier answer, please get professional help promptly. That having been said, the most likely cause of the swelling is a dental infection associated with an abcessed (infected) tooth. When a tooth becomes infected, the body tries to fight off the infection by creating a barrier around the germs leaking out of the tooth (out of the root of the tooth). This barrier is the cyst under the root of the tooth and there may or may not be pain associated with the cyst. However, at this point the infection needs to be treated and the treatment would be root canal therapy or extraction of the infected tooth. An untreated dental infection might be likened to a landmine in that it could explode (break free of the cyst wall) and invade the surrounding tissues (much as you described in your question) causing great swelling, pain and as explained earlier possibly death. There are other possible causes of the pain and swelling you described that would be best diagnosed by your dentist or physician. Please don't hold off seeking professional help.