I'm not always who I seem to be. At some points people get onto my name without my permission because they somehow found out my password. So if I say anything stupid it isn't me and I'm not lying!!!!!!!
If you're on a public computer, make sure you logout.
If someone else goes on, you're automaticly in. I wont hesitate to suspend your account or even ban the IP... even if it isn't you.
Have you thought of maybe changing your password?!? That seems the most logical step to me...