I really have a bad tooth in the back, headache, whole side of my face hurts, but I don't have dental insurance and on a fixed income. Where can I get help?

Is there a dental school in your area?

Here is info on some free clinics in different states. People typically drive long distances to reach these clinics. If you search enough you may find some fairly close.
Free Dental Clinics » Free Dental Work | www.freedentalclinics.net

Here's another one that I saw a news article about this AM.
Dentistry From The Heart

You have a abcess meaning your tooth is infected what you can do is go to the emergency room asap and get some medication to take for the pain and infection then go in to one of the local community dental centers that accept a sliding scale fee which is based off your income, most likely they will remove the tooth instead of a root canal and cap/crown so you may loose the tooth if you can not afford it. Let me know if that helps. In the mean time never wait because an abcess can be deadly.

Yes it can cause that, plus infections and other more serious medical conditions if not taken care of. Often one can get it pulled for 200 or less and sometimes cheaper if you talk with the dentist. Many have credit plans