Syphilis or Herpes depending. Syph is fairly rare these days in industrialized countries, but PRC is not one of those. Both have similar incubation cycles.
I'm not an MD, but chances are it's Herpes. Much more common microbes, but if you had it, it would be a series of vesicles (blisters) and not just one.
Syph is almost impossible via BBBJ. I disagree with SF City Clinic's chart. Either HSV-I or II is possible via orogenital contact. They know this. Syph is much more unlikely because the WG would have had to have the microbe in her throat. Not impossible, but that's not the usual parking place for Syph.
I am collecting some Chinese solo girl striptease DVDs right now. In 2 or 3 of the segments, I've seen girls model with open genital warts or Herpes vesicles. Really nasty looking stuff. Yeah, shaking my head....
Good luck, and hope there's no SO in your life you may have to explain this to.