What Causes A Vaginal Blister After Intercourse?

hello. My girlfriend is complaining about a very small blister of some sorts on her vagina. I took her virginity about 6 months ago. My fear is that it might be herpes but I have never had any thing close to any signs of this. I have not had sex with anyone before her in over two years. I was tested five years prior to that and have not had unprotected sex after I was tested. She is the only person I have had unprotected sex with. Is it possible that this is something besides herpes?