Red rash on 8 month olds chest?

Last night my 8 month old son had a few red bumps on his chest/legs. I didnt think anything of it because he gets little red bumps on random parts of his body here and there (born with a really bad newborn rash that looked more like welts!) but anyway, today the bumps didnt go away, which most of the time his normal bumps go away over night, and late this after noon the one on his chest is now a solid red rash about 2 inches wide and 1/2 in tall. Im trying not to get to worried but the rash isnt getting any better (lighter/fading) or worse. Im calling the drs asap in the morning but just wondering if anyone else has ever had this happen to their baby and what it was...thank u for reading!