How do you know someone is using your internet? The only way someone can use your internet after you changed your password is if they watched you or they have a keylogger on your system. What facts do you have to back up your thinking that someone is using your internet?
Check your routers documentation on how to look up what clients are attached to the router, most routers have this option. How many internet connected machines do you have? Another possibility is that you could be infected or you have programs installed and running that require internet access. Have you ran and malware or virus programs lately to check for infections.