There could be a few reasons here:
1) Have you been logging in from other computers?
2) Did you tell your browser to remember your login information?
3) Did you give your password out to anyone? (never do this..ever)

Some solutions:
1) Make sure you logout of your account from any other computer if you do login from another.
2) Never tell your browser to remember your login information, you can keep your passwords in notepad or something to remember them.
3) Don't give your password out!