If you have an unsecured wireless network - then another computer can log on and it might be assigned the ip address that you last used. Also some Internet Providers do not assign permanent IP addresses. With more details we can't help you.
Yes, agree with xpcman. Most likely your router acts as DHCP server that would assign dynamic IP to the computers which are connected to network. You could check your computer, most likely your network adapter is configured to obtain IP address automatically too.
You could try to locate that computer's MAC address on wireless router management page and proceed to block it (MAC address filtering) if it's not authorized user.