Allergic reaction sperm

Question, I've been sleeping with the same guy for almost 4 years. He often enjoys cuming on me. Today we did the deed twice, but for some reason the second time his cum seems to have given me a minor rash - On my tummy where he came as well as on my hand that helped him out. Has this happened to anyone before? Anyone got any ideas? Why did my skin react the second time and not the first? Help a sister out?