Pain Under Crown Months After Root Canal?

Though it is rare, there are times when a tooth that has had a root canal can produce discomfort.  One of several things could be happening.

1.  The tooth has an accessory (extra) nerve that is small and perhaps was not found.

2.  The tooth has a crack that is usually between the roots.

3.  The existing crown may not be fitting as well as it could.  

I would have your dentist do an evaluation and take an X-ray.  He/she can then give you  possible causes of the pain and solutions to correct it.  Good luck.   

If pain after placing a crown on a root canal tooth, upon chewing, the best dental treatment is to see your dentist for possible bite (occlusal) adjustment, and an xray. Usually after bite adjustment the pain subsides, however if the root canal was not properly done, usually after placing a new porcelain crown, especially the more stronger porcelain crowns, such as E max, or Zeronia crown,the tooth can flare up, and even cause infection. Meaning a tooth that had no pain or discomfort, got agitated due to new biting forces. An expert dentist must always anticipate this problem when crowning a tooth with unsatisfactory root canal, so that is why he will usually refer you to an Endodontist to redo an unsatisfactory root canal prior to placing a new crown.If You do have sensitivity to hot and cold, it is very possible that an adjacent tooth or an opposing tooth may need dental treatment, in which this can be verified by xray, and clinical examination.

Hard to say what is happening.  It is possible that the tooth is cracked and not savable.  But before you extract it, you should see what is up.  Best to be seen by an endodontist.

It is possible that one of the canals was missed when the root canal was done if the tooth is sensitive to cold.  It is also possible that an adjacent tooth is causing the sensitivity.  It can be very hard to localize pain from cold because of the type of nerve fiber that perceives the pain.  

You would need a 3D Cone Beam Scan to determine if there is another canal that might have been missed during the root canal treatment or there is a root fracture. Regular x-rays is two dimensional and sometimes the dentist will miss the additional canal during treatment.  
Typical fracture does not have throbbing pain. Usually one cannot bite down on it. Or it hurts upon releasing from the bite force.  Throbbing pain indicates either a very advanced fracture or beginning of another root infection.