The material used for refilling is harder than Amalgam so the dentist has to be more precise with the alignment, because if it isn't properly ground down so the teeth fit together just right, it will cause pain or headaches with chewing. (Just learned that today.)
Also, remember the 7, 14, 21 day thing. My pain suddenly became excruciating on day 14, which scared me because I had forgotten about that, but then the next day was better and continued to improve. Today is day 21 for me and sure enough they acted up again today. I'm sure they'll be better tomorrow.