What if the tooth beside the tooth you had a root canal on hurts really bad?

Have it examined: Could be that tooth, could be referred pain from the adjacent tooth... Hard to tell over the internet.

Get a diagnosis: See an endodontist and they will pulp test, percuss, do thermal tests and palate the root ends on all teeth in the area. A proper diagnosis results.It could be a problem with the root canal tooth or another problem.

Call dentist: That tooth could also need attention or it's a referred pain still from the tooth that had the root canal. Discuss with your dentist your continuing pain. When was the root canal done? Just recently or several months ago? Is the pain similar to what it was prior to the root canal or has it changed? These are important questions that need answers to give accurate diagnosis.

Problem: Not normal. The other tooth may have a problem. See DDS who did root canal ASAP.