Pain after a filling...?

Pain after three weeks is pretty uncommon and the fact that it is getting worse with time and that it was a "fairly deep cavity" leads me to believe that this tooth is probably going through a necrotic process, the nerve is hypersensitive and is "Dying" The pain will continue to worsen and eventually you will have an abcessed tooth. You need to get into your dentist and have him/her look at the tooth. If the pain persists then a root c will probably be necessary. If you are taking Ibuprofen and it is helping continue to take it until you get in to see the DDS. The other possibilities are that the bite could be a little off from the new filling or if it was a compostie filling that their could be a void in the filling and they just need to repack the tooth. If this is a top molar it could be a sinus infection that is causing your pain as well. If your symptoms are pain with cold/hot/throbbing/constant, then a root c is probably in your future.