Last year I had a root canal done on tooth #14. The tooth has a post and cap. Every now and then the tooth would flare up, and I would associate the pain with a gum issue between #14 and #15. I would take some antbiotics and it would go away. This is the third time the tooth has flared up in the past year! But this time it's different. I have throbbing pain on tooth #14 when I eat or drink hot and cold. The endodontist says it's IMPOSSIBLE to feel sensitive to hot and cold when I had a root canal and says the pain MUST be coming from a tooth on the bottom with a cavity. I told him that I am not stupid, I can feel the top tooth THROBBING after hot and cold foods or drinks. I have viewed my x-ray and I know that 3 canals were cleaned out. Is it possible that there could be a 4th canal? If so, why haven't I felt the sensitivity before? I am worried that I will waste 700$ on a tooth that I might have to pull out after all. Is it possible for a root canaled tooth to get infected again? Or could my problem be a periodontal issue? If it is would I be feeling hot and cold sensitivity? By the way, my x-ray shows no sign of infection even though I have pain, throbbing, and headaches only on that side of my head. I am currently taking amoxicillin, motrin 800 mg, and using peridex mouthwash. All of which is not much help. Thank you so much for your help, I will appreciate an answer of any kind.