rash caused by semen

Okay so this really odd. When me and my ex would have sex he would sometimes cum on me. Every time his semen touched me it would start to burn and it would itch and just hurt. I thought I was just allergic to semen which is rare but can happen. Also if he would pre cum and then not wipe away the excess cum left on his dick and then stick it back in me it wouldn't hurt right away but my vagina would be very tender and swollen after finishing. I thought that was just because we went pretty rough and he was not small. Anyways I have gotten into a new relationship and he ejaculated all up my front but it did not hurt me at all. It left no marks or anything like I was afraid of. So this rules out me being allergic to semen since I had no reaction to the other guys semen. I was wondering what you ladies thought could be the issue or if that has ever happened to you?