What you describe might be herpes, but could just as easily be a fungus or candida/yeast infection (which you could have got from her). Have your doctor take a look at it if it comes back in that area or if it persists.
A point of clarification: Although chicken pox is caused by a type of herpes virus, it's not the same type of herpes virus that causes cold sores or genital herpes, which are caused by Herpes Simplex Virus (HSV) types 1 and 2.
Most of the population has HSV-1 (usually around the mouth in the form of cold sores or fever blisters), so it's not surprising you and she would test positive for it. However, a blood test can tell you IF you have it but not WHERE it will break out and cause sores or symptoms. Most people have HSV-1 orally, but if you already have it there, a blood test won't tell you if you also have it genitally. For that you'd need to get a swab test done by a doctor (preferably within the first 24-48 hours after symtoms appear).
It may be too late to get your current symptoms swabbed for herpes, but you could try it if you can get to a doctor ASAP. Also, a visit to the doctor now could possibly determine if it's simply yeast or possibly herpes.
Genital herpes is usually HSV-2 but it's also possible to get HSV-1 there. This usually happens when someone with oral HSV-1 is shedding the virus and performs oral sex, which can transfer it to the recipient's genitals. This can happen even when the person already has it orally (i.e., they would then have it in more than one place). If someone then has intercourse with that person, they can get HSV-1 genitally too. There is also the possibility of transfering oral herpes to your own genitals if you touch your mouth (when you're contatious) and then touch your genitals.
So there are the possibilities of how it might have gotten to your genitals...IF that's what it is. However, it usually takes at least a few days for herpes to "take hold" in a new location, so it's unlikely that you got it from her on that weekend (if that's what it is).
However, if she had a yeast infection, I think you could show signs of it fairly quickly like that, but I'm not 100% sure of the incubation time for yeast infections. I once caught a yeast infection from a partner who had it and I *think* the symptoms appeared the next day. But your doctor could probably tell you if it's yeast or herpes or something else. If it is a yeast infection, there is medication you can take that will clear it right up.
I hope that helps. Good luck! (Sounds like the situation brought the two of you back together, which is great!

I hope it lasts.)
TheOneInFour