Pain Under Crown Months After Root Canal?

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.