Why do my teeth hurt after i got a filling but they didn't hurt before?

Sensitivity or hurt?: If it hurts, the cavity may be too deep and close to the nerve. If this occur, root canal may be needed. The filling could be high and thus the malocclusion causes pain.. If this occur, adjust the filling by grinding. If it is sensitive, allow couple days to see if the sensitivity will go away. If not, open margin or microleakage may occur and this requires redo of the filling.

Surgical procedure: Cavities form slowly and do not cause symptoms in the earlier stages. When you have a filling done, it is actually a surgical procedure on the tooth, which can cause the nerve of the tooth to become inflamed. This is usually mild, temporary and of no consequence. Occasionally, the tooth may require an adjustment or further treatment. Let your dentist evaluate it.

Various reasons: Most fillings are still a repair to an injury(decay or fracture from example) and your tooth may require some time to heal.(even though your chest may have been fine before, it's going to be sore after open heart surgery<g>). There may also be adjustments that need to be made to the restoration( adjust bite, or redo filling) and the nerve on the tooth may need additional therapy as well.

Tooth pain: Tooth is a live tissue, by working on it you traumatize it and it needs to heal. When you have a surgery, you stay away from the area til it heals, it is hard to stay away from your teeth. Bite adjustment is important as well as the more extensive the problem the more the trauma. If done with bonding, it is very technique sensitive, so if it is nit done correctly can result in post pain.