Any skin condition can, of course, have many many causes.
It could be a reaction to the detergent, but if you haven't had those types of reactions before I wouldn't chalk it up to that.
Given that your girlfriend does have HSV, and you have been having unprotected sex... it could be herpes. People generally take Valtrex (and other medications) for 2 reasons: 1) to reduce their outbreaks in severity and duration (suppression), and 2) to reduce the risk of transmitting it to others via asymptomatic shedding (when the virus is active on the skin and can be passed to someone else, but you have no symptoms).
It's possible to be intimate w/ someone w/ herpese for a long time w/o ever contracting it yourself. It's also possible to get it the first time you come in contact with it.
So... worst case scenario: you recently acquired it from her, which is why you're just now having these symptoms. If this is the case, your body hasn't yet developed the antibodies which will show up in the blood test.
(There are 2 ways to diagnose: by blood test, testing for antibodies, and by culture if you have an actual open lesion that the doctors can swab and test).
I was diagnosed by culture from a lesion during my first outbreak. The blood test I had done came back negative for both HSV-1 and HSV-2. 12 weeks later I got tested again, and came back positive for HSV-1 (genital). I got it from unprotected oral sex w/ my ex who got coldsores, but who was NOT having an outbreak at the time (asymptomatic shedding).
Anyway... it could be herpes, or it could be something else. You're understandably worried, but it's not the end of the world (even if it feels like that now). If it is some STD, it will probably change your life--and it will be challenging at times, but as you gain perspective and learn how it affects your body, it gets easier.
Do some reading online, educate yourself, and get tested for other stuff too. Your girlfriend should probably get tested too.
Then, maybe consider using protection w/ your girlfriend until you figure out what this is... b/c if it's not herpes, you want to reduce the risk of passing something else on.
Anyway, best of luck. Obviously I hope it's just a rash, but if it's not, you're in good company here. And--silver lining, your girlfriend already has it which should make it easier going forward.