If you haven't been sexually active at all with anyone else - no oral included in this - then sexually speaking, the only thing this could be, and it doesn't sound like it, is genital herpes type 1 if she has oral herpes and performed oral sex on you.
However, this could be SO many different things - a fungal infection like yeast or jock itch, a reaction to shaving, infected hair follicles, an allergic reaction to something - new laundry detergent, new soap, etc. You should definitely get it checked out to make sure it isn't anything that requires treatment. It doesn't sound like herpes, so don't be too scared, okay?