I want to share my recent experience in the hopes it'll help others.
Over the last ten months I have had three outbreaks of painful, pussy sores on my vulva. The first time I was diagnosed with herpes and given antivirals, but the test came back inconclusive and the doctor said that given my history, herpes was unlikely and it was probably some kind of random irritation. When the second outbreak started I was tested for herpes again, along with all of the other standard STI tests. Everything came back negative so the doctor put me on antibiotics. The third time, which happened just this month, my doctor still didn't know what caused it so he suggested I got to the STI clinic because they might recognize it as some kind of rare or exotic STI that he was not familiar with.
I went to the STI clinic where they diagnosed me with herpes and put me on antivirals. The test results came back negative and the clinic staff responded by testing me for herpes again and extending my antiviral prescription.
At the same time I got a really bad flu, which was probably unrelated to the outbreak. However, it landed me in the Emergency room in the hospital, and the doctor who saw me referred me to a dermatologist to check out the sores. Armed with several, recent negative test results for herpes and other STIs, I went to the dermatologist who immediately recognized it at Idiopathic Genital Ulerations/Aphthae. Basically, they are giant canker sores on your genitals. By the time I saw the dermatologist I had two sores that were both about 2cm across. At that point I could not urinate without hitting at least an 8 on the pain scale, and I was unable to sit or walk comfortably.
I was prescribed Prednisone and it seems to be helping.
I am posting this because my husband and I did a fair amount of research and this particular condition did not come up anywhere. I was frustrated with the STI clinic because they ignored the fact that I tested negative for herpes three times in a month (plus other negative results from the last couple years) and instead of investigating further, they just continued to retest me and give me more useless drugs while my condition worsened to the point of debilitation. So if you have something that looks like this, and you've tested negative for STIs, please see a dermatologist. This is a rare enough condition that I think most GPs would not recognize it.