It's very unlikely that the swab test result is false positive(unless the lab has made a mistake in typing the result paper which I doubt it happened ), they can be false negative but it's really hard to grow the virus out of nothing. So your gf has genital herpes. But did they tell you guys which type she has? you'd better ask for the type. If it's type 1 then you'll know that she probably got it from you performing oral sex on her. I have genital herpes type 1 from the same experience. And the fact that she never gets cold sores makes it even more likely to get type 1 on her genitals because if she hadn't been exposed to it before then she didn't have anti bodies against it to fight it off.
About your question regarding the site of the outbreak...you don't have to receive anal sex to outbreak on your anal area. It's just the virus that picks the nerves path to the skin and you can basically get outbreaks any where in the boxer shorts area. It doesn't mean that the virus has entered through that area.
About you and not having cold sore at the time of the oral sex: You can shed the virus on your mouth at any time even without getting a cold sore. You know what, None of my boy friends have ever had a cold sore in their lives but I got type 1 down there. That's why I'm sure one of them had been infected without any symptoms.
Also it is possible to get a herpes outbreak and yeast infection at the same time.
By the way, She can ask for a type specific herpes blood test to see what type she has id they haven't typed the culture result already but she needs to wait for about 3 months to develop the anti body in her blood to show up, other wise it' gonna be false negative.