Hi, I was diagnosed 5 years ago with HSV 2. I was having outbreaks off and on 2 months before I got officially diagnosed. It was localized around my vaginal area where my partner and I had been having sex. I didn't have an outbreak when I first went in to get tested. I assumed, like you, that herpes was included with all the STI testing. It was a shock when I went back in to the doctors (this time with an outbreak) and was visually diagnosed. So I feel you.
Outbreaks can occur anywhere on the vulva. However, since you guys were doing anal it makes sense that it's localized to that area currently. I would suggest going in to get medication from your doctor. That will help with the pain and getting rid of the blistering. I always know when I'm about to get an outbreak with a "tingling" feeling on the back of my thighs and up my butt, if you start taking the medication, it can help prevent any painful blistering.
Let me know if you have any other questions. You're not alone in this.