X-ray does not show any infection.
Why Would an Old Root Canal Start Causing Pain?
Doctor Answers 2
Why is my old root canal now hurting me?
There are several reasons. The first of which is that the root canal treatment may be failing and need retreatment or possibly a minor surgery to save the tooth. Root canals are very successful, exceeding 90% today.
Another reason is that the tooth may have cracked under the crown...you did have a crown done over the root canal tooth, didn't you? Most root canal teeth do need restorations that are stronger than just fillings to strengthen the weaker root canal tooth. Root canaled teeth fracture easier and more frequently non-root canal teeth.
A third reason is that you may have a gum problem or periodontal abscess around that tooth, that has no relationship to the root canal treatment.
A fourth reason is that you may have had some movement of your teeth (most likely due to periodontal disease) and you are hitting harder on the root canal tooth than the others.
OK, you got the run down of possibilities. What did your dentist say after the x-ray and he examined your tooth? If he can't give you an answer, then suggest you have your tooth evaluated by an endodontist (root canal specialist).
Old Root canal Causing Pain
There are several possibilities for the circumstance you describe. Root canals can fail over time and an abscess can develop or get larger. There could be decay that has developed under/around the restoration causing pain. A root canal tooth is also vulnerable to fracture over time. It could also be referred pain from another structure including adjacent teeth or even your jaw joint. You will need an - x-ray, thorough exam and history taking to determine the source of pain and possible infection. If indeed it is a root canal that is bad, it would be best to remove the tooth and replace it with an implant