I would be curious to know if the tooth that had a root canal performed on it and now is causing you pain, could have another canal that was not identified on x-rays?
I ask this question because I was one of those patients. I had a tooth that needed a root canal and one was performed. Months went by and all was fine. Same tooth started giving me pain and had swelling again and went to the dentist who performed a second root canal. Months went by, same tooth became a issue once again. Went to dentist and he took a x-ray and examined me. He did not see any swelling or infection based on his examination and x-rays. However, he knows me well and knows that I know my body well enough to know the tooth was infected and needed a root canal. He referred me to a Endo and the endo took special x-rays and found I had a third canal in this took that was not visible on a regular x-ray that is also known as a buckle. The doctor said it is rare to have more than two canals in one tooth. In my case, especially being a tiny front tooth. After finding the buckle and cleaning the third canal rhe tooth was fine and healed.