Infections occur in the jaw approximately 10-15% following root canal therapy so your condition is not uncommon. It can occur many years after the initial treatment was completed well.
Usual treatment consists of re-doing the root canal if possible, cleaning the infection directly around the root with surgery, or removing the tooth.
In general, endodontists are very well trained and capable or treating your condition. If you have any doubts, I suggest you seek a second opinion consultation from an Oral Surgeon. They see these infections everyday.
Information contained within this reply is intended solely for general educational purposes and is not intended nor implied to be a medical diagnosis or treatment recommendation. This is not a substitute for professional medical advice relative to your specific medical condition or question. Always seek the advice of your own doctor for medical condition. Only your doctor can provide specific diagnoses and therapies.