Expert Q & A Answers About semen allergy

My girlfriend gets a rash after sex. Is she allergic to my ejaculate?

On a few occasions when I get some ejaculate on my girlfriend’s thighs or after intercourse, she gets a red rash like poison ivy. It doesn’t happen right away, but overnight and it goes away after a couple of days. Is it something in my diet? Medications? Or is she simply allergic to my ejaculate? The only allergy she knows she has is to sulfites...does my ejaculate contain them? We are both 24 years old, healthy, STD-free students, and we’ve been together four years.