Are you the only one using your connection? If you know of other people using your connection (i.e. room mates, family members, etc.), then ask them. If you're the only one that's supposed to be using it, then I would change your wireless password, as well as your SSID (optional, but recommended).
Also, if your router has WPS enabled, disable it. This is a super vulnerable authentication system for your router if it's enabled.