I was signed on on my bf's laptop, and when I came home the site wouldn't let me on because I was signed in elsewhere...I had to have him go on my account and sign me out so i could sign on at home...I rarely ever sign out between places (except maybe facebook).
I have it happen every once in a while, and it's been going on for months. I wasn't sure what the issue was either, kind of freaked me out at first, but it eventually fixes itself.