Small red dot on my sons arm?

My son is 4 months old and when I was giving him his bath this morning, I noticed a small red spot on his arm. It isn't raised and is not a pimple. It must have appeared overnight because he did not have it yesterday. He isn't really acting funny or anything, I'm just curious as to what it may be. I was thinking maybe the start of chicken pox but I don't know if they only happen in clusters.