I have a yellow, open ulcer at the back of my mouth. i have an infected filling, and i am on antibiotics. will the antibiotics heal the ulcer? it appeared yesterday. there is swelling and there has been pain for three days?

If the bacteria causing the infection is sensitive to the antibiotic, then there is a chance the ulcer will heal. But, the best thing you can do is to see a dentist ASAP. If the filling is infected, it will need to be replaced. The dentist can evaluate the infection and determine the best course of action. The infection is unlikely to heal with antibiotics alone-seeing the dentist ASAP is needed.