my daughter got the same red color 粒粒 on her neck when she was < 1 month old. This happened becasue I once used too much baby oil on the bath. I went to see dr., and applied steroid for 2-3 days, and recovered then. Dr. said it's only allergy, may be to a particular substance, say baby oil.
I think you should see a dr., but don't be too worry