hard to tell from the pics.
but if it happened after a day outdoors, i would say it's good old fashioned sun rash. especially since your dog is white with fair skin (more sensitive to UV light).
if that's the case, i would wash with something mild like baby wash and apply a small amount of aloe vera. maybe a cold pack for 15min every few hours.
if it turns out to be an infection, the vet will have something for it. good luck.
~ jo and Gotti Boi (aka Thunder Thighs, aka Jughead, aka The Doo Doo Monster, aka Sexual Chocolate, aka Mister Smiley, aka Sofa King)