M.M.
My first thought is that he is in a growing spurt. Usually they eat eat eat, to build up those calories and fat, and then they sleep sleep sleep (which is when they grow grow grow). Hope you haven't stocked up on his current diaper size! LOL Sometimes babies spit up a good amount if they have had enough to eat, kinda like change for a dollar. He only needed 75 c and took a dollar, well, here's your quarter back (and since he was stored up for growing this seems more than likely to me). :P
Glad to see that are going to the doc though, they are the experts and know about things (and check other vitals) that we don't know right now (like what that rash if from).