Very creepy

I just tried to access my page on nanowrimo and got this message:

"The login cookie that your browser just provided is incorrect. One possible cause of this error is that your web browser cookies have been stolen and used by someone else to impersonate you at this site."

Why didn't they suggest another possible cause? If there's one possible cause, I think there must be another one.

Anyone know what it might be? Or, if you're stealing my identity, I'm warning you, it won't get you very far, so please give it back.