I've no ideas for you --- and 53 other folks looked at this post and thought the same thing, apparently.
On the belly, Bo gets little dot like bumps which aren't zits at all but little hives; reactions to chicken. Cut grass gives a similar -- but larger -- bump. He does not however, get bumps on his neck - ever. So that one bothers me a bit. Could be food, could be the soap or the plastic loo --- could be lots of things or nothing at all. That's the supreme challenge here and you're doing it right via the process of elimination. A look from the vet to make sure it's not something serious and then, start watching everything that goes in / on / near the pup. Good luck. We've all been thru something similar, I'm certain.