rash on infant... measles? chickenpox?

My 5 month old son just started breaking out on his back, back of his head, and now on his chest with red rashes.  The rashes are along with small, what seem to be, pimples.  He has no fever and it more aggitated than anything.  He had 4 shots less than a week ago so we are assuming it might be a reaction, since that's kind of what it appears to be.  I have pictures i can email or send for visual..  Please advise.  Thanks.