Why do I have pain when chewing after having a composite filling?

Whenever we work on a tooth it produces trauma to the nerve.  The body's response to the trauma is to pump more blood to the area.  This puts more pressure on the nerve and makes it respond more.  If the bite is adjusted so that all the teeth touch at the same time and that tooth is not touching early,  then the pain should begin to subside, but it will take time.  It should take anywhere from a few days to a few weeks.  Avoid that side and let the tooth rest as much as possible for a week and then try it.  If it is no better in a month, or starts to get worse, you should return to your dentist for evaluation.

It can be as simple as your tooth is settling with the new filling.  It is very common to have sensitivity with white fillings. Another possibility is that your dentist or an Extended Function Dental Assistant (EFDA) may not have compacted your new white filling thorough enough where little voids (holes) are left inside the tooth.  Another possibility can be the bonding (glue) material that your dentist used to activate the new filling to bond to the tooth structure.  Some older generation of bonding material are known for sensitivity versus the newer generation of bonding materials.  Lastly, maybe the new filling was too deep near or hitting the nerve.  Either or, if the pain persists, then you need to go back and have it checked.

Thank you for a great question. When there is a discomfort on chewing after a tooth colored filling, it should be a temporary event and will disapper within a short time, usually a few days. I always advise my patients to be aware of such possibility and if it does go away, to come back no longer then 1 week to reevalute.

For the first few weeks after the procedure, any cold or chewing sensitivity maybe normal if the bite has already been verified to be correct.  This sensitivity is from the procedure itself and your bodies natural inflammatory process.
After a few weeks if the pain has not subsided, the composite may need to be redone.  There're a number of things that can cause this :
-some moisture from your breath or saliva inadvetantly contaminated the bond, unknown to the dentist
-the bonding did not get set (cured) all the way
-the tooth may have been over-dryed before the bonding 
The good news is, it's an easy revision if needed.  It happens to the best dentists and the nicest patents all the time !