My Son Had The Same Reaction As Described Below Only

My son had the same reaction as described below only several white spots or a white spotty area with redness and a small amount of raised texture to the skin that turned white. All of the answers were in regard to a white spot on the INSIDE of the mouth, this was also the OUTSIDE of the cheek in the area where the tooth was filled. The dentist thought it was an allergic reaction perhaps to the nitrous tubing, Gave benedryl and it went away after an hour but was there again faintly in the am. Any other thoughts as to what type of reaction this could be?