Far as I know you can't catch oral herpes by walking barefoot in the shower.
I'd guess the more likely candidate is your girlfriend who happens to have cold sores, which can be transmitted when the person who has it is asymptomatic.
Here's some info about the different types of herpes:
http://www.nlm.nih.gov/medlineplus/e...cle/001324.htm
http://www.mckinley.uiuc.edu/handout.../herpsimp.html
There are also non-herpetic mouth sores called apthous ulcers:
http://www.animated-teeth.com/canker...nker_sores.htm
However, best to be safe and assume it's probably herpes and probably contagious, so be careful about oral sex, sharing drinks, etc.