Burping is probably the culprit then -- my bad. (he's not on solids, and I often skip burping thinking he doesn't need it.)
I've been working on naps for 2 months now, but only 7 days on BW. Since we started BW, and since he learned to fall asleep independently (yay!) it's been 20 minute wake-ups consistently.
On the Naps board, a few of us are speculating that our little ones might have recurring reflux (since they had it when they were littler) that is rearing during nap time. Spit up, hurting faces, acid smell, etc. Not sure if I hope it's true or not, poor thing.