Possible Semen Allergy

Patient: Everytime my boyfriend ejaculates on me (my stomach, back etc.) I get a red, itchy rash. I researched on the internet about the possibility of me being allergic to his semen, but I don’t get any reaction at all when he ejaculates inside me. (We no longer use condoms as we are both STD free and I am on the contraceptive pill). If I wash it off straight away then I am sometimes OK however it can somewhat ruin the moment having to have a shower as soon as we are finished. What could be causing this?