False positives are much rarer than false negatives, but they do
happen.  Just off the top of my head, I'd say that false positives
are only likely in around a one or two percent range.  It's possible
that a lab could screw up, somehow, I suppose.  

The way you've laid it out there, I would say it's more likely that
your girlfriend's results are the wrong ones, which isn't to say
that she just might have HSV after all.  But a positive result is
much more trustworthy than a negative one.  Depending on the test
she had, and how soon after she might have become infected, the
chances for a false negative can go all the way up to 50%.  It is
recommended for most of the modern blood tests that the testee wait
twelve to sixteen weeks after possible exposure before submitting a
blood sample.  Since the tests search for the antibodies that the
body manufactures to fight the virus, and the body only gradually
supplies those antibodies, a test given too early may fail to detect
them.

It's also very possible, even if you've had HSV for a while, that
you haven't transferred it to your girlfriend.  Many, many couples
don't transfer the virus.

Really, the only way to settle your mind, is to go have a second
test, perhaps at another facility, if you don't quite trust your
present doc.  Many walk-in clinics can do the work for you.

Hope that helps,

Mike

False positives are rare. It's more common to have a false negative.

Angela