I've been having headaches jaw pain pain under chin and neck pain I'm scared I have tetnus what are they symptoms of that? How would I know if I had it? My legs and back have been hurting as well

Can't open mouth. Tetanus is pretty rare and requires that you had a cut and it was infected with spores typically found in dirt. It is very severe, you become unable to open your mouth or even swallow your saliva, and muscle spasms you can't control. If you might have been exposed (had a cut) then you should get checked and at least have a booster shot.
Not likely. Not likely to have tetanus. Most of us have had a tetanus shot at one time. You may have numerous other diseases or afflictions. Could be a simple virus causing these body aches. Call your doctor or go to a clinic for an exam.