Whys my tooth sensitive after a tooth filling a year later?

If the restoration is very deep it is possible that the nerve is irreversibly inflamed. The tooth may also have a fracture. If it is not sensitive to cold food, but hurts when you bite down on it, the tooth is most likely non-vital and will need root canal therapy. Have your dentist evaluate it.

Did your dentist tell you at time of filling that this is a deep cavity and close to the nerve? If so than the sensitivity is normal and a new x-ray is indicated and a possible root canal is necessary or a re-filling.