My Boyfriend And I Had Sex The Other Day First

My boyfriend and i had sex the other day first time in a while well as soon as we got done he said that he was sore and was looking at this certain area around the base his penis when i looked i seen a red area the next day there were blisters and he swears he hasn't cheated but i know something is odd because it looks like a std and i have no symptoms and we been together 3 years and how could that just appear all of a sudden? I would like to know what it could be??