Neck rash - from drool or allergies???

Has anyone experienced a similar rash on their LO? It started out as just a little redness (no bumps) in between the rolls on his neck, but has since spread to his chest and become a bit bumpy over the last week. He's started to drool a lot more, so I first associated it with the extra wetness, but now I'm not sure. I tried Aquaphor which seems to help a little, but not much. And cornstarch almost seemed to make it worse.