i just had sex with a girl and she told me that she has herpes after we had had sex. she was not experiancing a breakout and says that she is very responsible about her medication. what are the chances that i have contracted the STD?

First of all, if she was "responsible" she would have told you before you had sex, and let you make the decision. I can't really tell you what your chances are, but I can tell you to keep a close eye on the area, and get checked. I've read that the herpes antibodies take around 3 months to show up in your blood, so you could wait 3 months and get a blood test.

If she is on supressive medication and was not having an outbreak, your chance of getting it is something like 2% per year....so chance from 1 time is very small...but not impossible.