To answer your first question, yes it's probably heat rash or even baby acne. Very normal in young babies. Try applying some of your expressed breast milk on the rash and let it dry. Also, stay away from over-perfumed skin care products which might aggravate the rash. But if it doesn't clear up in a week or two, I suggest getting a second opinion from another doctor. As for your second question, again very normal for breastfed babies to not poop for days. My first-born could go for up to a week without pooping when he was breastfeeding! But once again, if your LO seems to be in a lot of discomfort or seems very bloated, check with the paed.
Mrs Ummi