Well, you have done a good job of getting checked out and ruling things out. Does not sound like an std (you've checked), hpv or any issues like that. How's your lubrication? I guess after months of this stuff, it would be possible that it would be hard to be totally turned on. Do you use an artificial lube that you may be reacting to? Or don't use any that causes you to then have the issue? Do you use condoms?
Whether or not you are using condoms could be a clue. It could be an allergy.
If you are using condoms, possibly you have developed an allergy to the kind you are using.
If you are not using condoms, there is a rare condition in which a woman develops an allergy to the proteins in a man's sperm. The long-lasting nature of your reaction seems to argue against sperm allergy, but if you are interested, below is more about it. You could tell by using a condom (if you haven't been) and seeing if you have the same reaction after sex.
"Sperm allergy, sometimes called semen allergy or seminal plasma hypersensitivity, is a rare allergic reaction to proteins found in a man’s semen. It mostly affects women.
"Some common symptoms of sperm allergy are redness, swelling, pain, itching, and a burning sensation in the vaginal area. Symptoms usually start about 10-30 minutes after contact with semen. They may not be confined to the vaginal area; they can occur in any area that has contact with semen, including the skin and the mouth. Symptoms can last for a few hours or a few days.
"For some women, the symptoms are localized – they stay in one main area. But for others, the symptoms can affect their whole body. They may have hives, swelling, or trouble breathing.
"Sperm allergy may be discovered the first time a woman has sex, but sometimes it happens after a woman has had other sexual partners with no allergic reaction. Sperm allergy may also occur with one partner but not another. Or, it may happen suddenly with a longtime partner.
"The condition is often misdiagnosed as vaginitis (inflammation of the vagina), a yeast infection, or a sexually-transmitted disease (STD) like herpes. One clue for diagnosis is condom use. If sperm allergy is present, the woman should not have any symptoms when she and her partner use a condom. The allergic reaction should only happen during unprotected sex."