Sometimes, a tooth may become sensitive after having a filling done.
Here are the most likely reasons!
The filling was really deep and close to the nerve of the tooth
A really deep filling is required when the decay had been left untreated for a long time. Eventually, tooth decay will grow into the nerve of the tooth and then the tooth will require a root canal or even extraction.
When it is close to the nerve, the nerve may become irritated from the chemicals and the removal of decay. Sometimes, special medication is placed onto the tooth to prevent the sensitivity and requirement of a root canal. Even with the medication, it is normal to expect some sensitivity to hot and cold foods for around a week. If it does last longer, it probably needs a root canal, because the nerve is starting to die.
Usually, the dentist will inform you of this at the appointment, so it's not a surprise.
There is a gap between the filling and the tooth
When placing a filling, air pockets are sometimes trapped in between the tooth and filling. When this happens, minor flexing of the filling will cause this air to be pumping against the tooth, which can cause sensitivity. In this kind of situation, it will only be sensitive when you bite on the tooth.
In general, this will pass over time, but it would be a good idea to see the dentist again to check on it.
The filling is too high
This should be obvious to you, it will feel like you're biting on that tooth much harder than the other teeth.
The dentist will check that the filling fits in your bite, right after placing the filling. Sometimes, after having your mouth open for long periods, you may not be biting as usual, so the check is inaccurate.
In this situation, see your dentist again so it can be adjusted. It won't require anaesthetic and is a fast procedure.
Feel free to leave any comments and questions!