If your partner indeed has recurrent yeast infections, and not something else, almost certainly it has nothing to do with you. Few if any cases of candida vulvovaginitis (the medical term) are acquired from partners. Although recurrent yeast infections have been cited as consequences of HIV infection, this actually is not a particularly common consequence of HIV; and to the extent it occurs, generally it is a sign of advanced immunodeficiency that occurs many years after acquiring HIV, not as part of a syndrome of early HIV infection. Add to that the low risk of men acquring HIV from any single episode of vaginal sex, and you really have nothing to be concerned about along these lines.
as you have learned, the STD most commonly confused with candida VVC (and vice versa) is herpes, and conceivably that could explain some or all of your partner's symptoms. However, the dime-sized "patch" doesn't particularly sound like herpes; and like HIV, the likelihood is very low that you acquired asymptomatic HSV from the exposures you describe (although not as low as for HIV). Indeed, if your girlfriend turns out to have herpes, she could well have acquired it from a former pertner, not necessarily from you--unless you are her only lifetime partner.
To answer your specific questions: 1) See above; fairly uncommon and generally with late HIV disease. 2) Yeasts are not acquired primarily by sex; all women carry yeasts some of the time either in their intestines/feces or vaginas, and sometimes they overgrow. 3) You probably can relax while you await your test results. It is unlikely you acquire either HIV or HSV during your lapse in sexual safety.
Good luck-- HHH, MD
Herpes, on the other hand, might be an issue, since it's often misdiagnosed as yeast - and because you describe seeing a patch that looked like cold sores. But even if it is, it doesn't mean you gave it to her; she could just as well have entered the relationship with it - in fact, both of you could. And if you have HSV1 orally, you could have given it to her genitally through oral.
Don't let your guilt take over and cause you to overreact to any of this. You really don't have anything to freak about. So you didn't use condoms with your other partner; that's not great, but you only had two encounters with her and the risks of contracting any STD during that time are pretty low. You'll be fine, and so will your gf :-)
You're the bomb. Good answer.
HHH, MD