My dr removed and replaced loose bracket. I felt so much pressure and pain. Dr said it's normal to feel pain coz teeth are still moving. Is that true?

Should subside. Within a day or so. The pressure is common and to be expected. However if it lasts more than 2 days, call your orthodontist and discuss with them.
Yes, it's true. The bone is sore due to the action of the braces and wires. You should be fine.
Possible. What he told you is true. There is a posss that there is something happening if there is a filling in the tooth. Residual caries(decay) is a condition that exists under a restoration. It many time does not show itself on the radiograph. If the discomfort does not subside see your general dentist. The contact of chewing may also have the tooth a little "Wiigged Out". The bite can be adjusted.