Although I wish that this was not so, sometimes patients do experience discomfort after having a filling done. Sometimes the pain is associated with a bite that needs to be adjusted. If the filling that was placed hits just slightly before the other teeth hit it can cause pain and sensitivity.
If a patient calls with pain after a filling is placed, I always ask if they bring their teeth together does that provoke the pain. If it does I ask them to return for a bite adjustment. Often this 30 second adjustment is all that is needed. The tooth becomes comfortable within 24-48 hrs.
Infrequently, the bite adjustment doesn't work and in that case if the pain is not severe, I will suggest giving the tooth some time to calm down(one week to one month). If the patient is still bothered at that point and there has not been an improvement I suggest taking the filling so that I may have a chance to reexamine the tooth without the filling.
After removing the filling, I study the tooth with a bright light and magnification and look for either internal cracks or for possible microscopic exposures of the nerve. If none are found I place a sedative 'temporary' filling to see whether that helps. If I do find an internal crack in the tooth that may be responsible for the patients sensitivity, I often recommend making a crown for the tooth and having it placed temporarily in the mouth to see if the tooth becomes 'happy'.
A small percentage of teeth even when all is said an done remain 'unhappy' and do require a root canal to remove the pulpal tissue. Root canals, when performed properly, have a very high rate of success and will give these patients back their 'happy' tooth.