I have had a root canal a year ago.  It suddenly flared up while I was traveling acroos the United States.  We had to stop in TX where an endodontist took care of me. He said I had a large abscess.  He gave me ic clindamycin which seem to heal my infection.  This was in May.  Suddenly two weeks ago it flared up again and my dentist put me o clindamycin again.  This time it is not working.  My endodontist told me that I had irreversible pulpitis and drilled holes in tooth and cleaned it out.  He also put medicine in it and thought it would be fine.  He is now out of town and the pain has returned.  Nothing is working and I will be out of meds tomorrow.  I DONT KNOW WHAT TO DO.
Thanks for your query, I have gone through your query, once the root canal treatment is done the pain should be there. So consult an oral physician to rule out any defect in RCT or whether any decay in the adjacent tooth causing pain. You need to take radiograph. If I am your treating doctor I would have suggested penicillin with metronidazole which is most effective and economical thank clindamycin. If there is defect in previously treated tooth it should be retreated or extracted. I hope my answer will help you, take care.