An allergy to certain proteins in semen, also known as human seminal plasma hypersensitivity. It's estimated that as many as one in twenty women have this allergy. This condition can often masquerade as a yeast infection or herpes infection. The cause isn't known.
The most common symptoms are localized burning, itching, pain, swelling, redness, or blisters forming within 30 minutes of exposure to semen, but usually the reaction occurs within five minutes of contact. Severe reactions can lead to trouble with breathing, anaphylactic shock, and even death in very rare cases.
If you think you have a semen allergy, you should talk to your doctor about it.