Sometimes people allow others to use their email accounts and information. Perhaps this applies here. Just seems strange that a girl you describe as being decent would be scouring the internet for sex. Most of the people who use sites such as those are either men, nymphos, or glaringly unattractive and despearate women. I have to assume from what you've said that your girlfriend was none of these. Are you in a position where you can still send her an email and ask? Is she in a position where she would respond to you? Seems to me like one of those things I'd want to know.
And I am shaking my fist at you for making me come out of my ghostly trolling mode that I savor so very, very much.