Blister in mouth?

Yesterday my son had a white nub on his gum and I thought it was a possible tooth coming in. Today he's been drooling a lot and fussy again I assumed it was a tooth. Tonight I looked in his mouth again and the nub looks more like a blister and has more white spots around it. Has anyone experienced something similar? He hasn't been around anyone with a cold sore or anything. I couldn't get a picture because he was moving too much. I'm really worried and calling the pediatrician tomorrow but figured I'd ask in case anyone has any advice.