Allergic to sperm

I've been withy boyfriend 4 years now and today was the first time I let him cum on my chest (can't stand the smell of it so never let him do it till now) I've gotten a red rash where it was. The rash started right after he cummed cn I have mild allergy to it. I have 2 kids and we used to have unprotected sex all the time at beginning of the relationship never had a rash or issues down there so I'm just surprised now