Is this a semen allergy?

My wife and I have had sex before and she has gotten semen on her chest and face, but this time she had some run down her leg after coitus, and in the exact path that it ran down, she has gotten a rash. It spread a little bit to her other leg (although we dont think very much, if any semen was there.) It hasnt spread to any other part of her body after 30 minutes. Is it possible to have a very mild Semen allergy? There are no blisters or any of the common side effects listed online. Any ideas?