My 7y.o. Son has a pink, bumpy rash on his foot, just one, on the sole. It started as a blister, then broke, a string of bumps moved 1 inch almost in a straight line. Then another blister broke, and the line moved another 1.5 inches away. The bumps look like poison ivy, but it is winter here. No other symptoms. I checked his hands thinking hand, foot and mouth, but he is not sick, is happy. What do you think?