Most people are unaware they have had hsv symptoms. Most people do get stmptoms to one degree or another. Some are minor and get passed off as something else. You being on the "look out" would have noticed if anything was unusual.
Now as far as you testing, the std world is kinda split on this topic. On one hand routine std screening seems like a great idea. That being said hsv2 igg blood tests are sensitive and low false positives are more common then we would like. This always sends the person into a tailspin. The problem is most Dr's are clueless about hsv and testing. So the test comes back barely numerically positive and Dr says "yep, you got herpes". When in reality they don't and should be given a confirmation test like thw western blot. In your case having only two exposures and her producing a neg test. Not to mention you've had no symptoms. That's all the evidence I would need. People tend not to lie when asked about stds by the way. I'd believe her and move on