Hello, If you have sensitivity  3 months after doing the filling, there are usually four probabilities. 1. The decay was deeper and had not been removed fully. 2. The filling is too close to the pulp (the nerves, blood vessels etc, inside the tooth) 3. The filling itself was not done properly (margins may not be intact, or inadequate technique) 4. You would have bitten on a stone or something very hard exactly on the filling  (the sensitivity decreases on its own after a few days in this case) Hope this helps. Regards

Either the filling is too deep or there could be decay inside . Best thing is to replace the filling and check. X-ray can be done as a guideline but clinical judgement would decide treatment