:shock:
years ago I did the same thing with a woman I was dating, though with me it wasn't intentional.
You see, everytime right before we'd get together for the evening or night, I would pop in an peppermint lifesaver or some other mint. I would also do this right before sex as well.
Well, my lady friend went through the same problem as you, with the itching and everything, only we couldn't figure out what it was from. Finally, after thinking it through for awhile, we realized that it only happened when I ate the mints, so I stopped doing it. Only, my lady friend started telling me that there was a difference in the feel of my oral play, and again we surmised that it was because of the mints - or, rather, the absence thereof. So I started using mints from that point on - and still do to this day - but my now wife thouroghly washes up immediately after sex, and that seems to keep the itching at pay.