Sounds like a bacterial infection.
I have this same issue on and off for a couple years now. I have also only been with one man, my husband of 3 years. I believe that the problem occurs for me when I am changing my tampon really frequently, wearing one over night, and having to wipe myself a lot. I have somewhat heavy periods and so these things are kind of unavoidable. It goes away once the irritation stops. I feel better knowing this is a common issue, but I’m still not sure why gynecologist are acting like they don’t know what the issue is. It seems pretty clear to me that having a lump of cotton taken in and out of your body causes irritation. If there is nothing they can do that’s fine, but atleast put people at ease by admitting what the issue is.
Some women are actually allergic to their partner's sperm - not all partners necessarily, but something doesn't "mix" well with that specific couple and the woman's body doesn't tolerate it - I think it's the protein or genetics. It's a real allergy and not just psychological. If you notice that it happens after ejaculation and not just around the time of your period, it could possibly be this.