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.
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.
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.

Related Questions

I had a root canal 2days ago. Still lot pain. My concerne is that I feel exectly the same way - cold... Hot... When touchs my tooth hurts really bad.?

Infection in bone. Root canal procedure clears/ cleans the tooth part of the infection, but there might be some peri apical infection in the bone which can take a few days to clear. Antibiotics and pain meds will help. Sensitivity to hot or cold might be with one or more adjoining teeth which might have some issues. Please have all your remaining teeth checked. Read more...
Another tooth. It is not unusual for a tooth to be sensitive after a root canal. Adjusting the bite should reduce the pain to touch. But hot and cold pain is probably coming from another tooth. Read more...
Was RCT completed? If the tooth had the rct completed, the nerve, blood supply & all tissue is removed from the canal(s) and therefore it is impossible to feel sensation to hot & cold from that tooth, although it is not uncommon to feel soreness to touch for 1-3 days. If the rct was only started & not completed yet, you could still feel sensitivity to pressure, cold &d hot. Discuss your symptoms with your dentist. Read more...
Multiple causes? It is possible that (1) the root canal treatment is not fully completed. (2) it is "completed" but a canal was "missed" and a nerve is still signaling hot / cold sensitivities. (3) you had two teeth in need of therapy and only one was treated . (4) the wrong tooth was treated. If your dentist (specialist or g.P. Is unable to determine the cause, get a second opinion from an endodontist. Read more...