I had a similar situation when I was diagnosed. I started seeing a guy, and two weeks after we had sex I ended up with HSV2. Now I had been tested a few months prior, and I had only two sexual partners after that time. I spoke to the other guy (a great friend of mine, someone I had sex with first), and he told me that he had been tested a few months ago as well. So that left me with one option, the man I was currently seeing. I was soo nervous because I thought he didn't know that he had this. That I would be breaking the news to him like my doctor did to me, and he would be devastated. That didn't happen.
He knew that he had the virus, and he chose not to disclose. I say this because anything is possible. I wouldn't assume she knows, but maybe she does. In his case the VA gave him some very terrible information on the virus, so he thought he could only spread it during an outbreak.
The conversation is hard. Very hard. But the faster you rip the bandaid off the better. :) I would just tell her you have something to discuss and tell her you just got diagnosed. The more honest, and upfront you are the better. If she doesn't have it she will appreciate that. Best of luck. You will be just fine.