that was cleared up by rebooting the router. If your router configuration has a "DHCP Lease" time, duration or similar.. value should be in seconds... set it to 14400 seconds (4 hours). this will keep your DHCP lease database fresh, and remove older entries that are no longer connected.
You could also set up individual reservations for the different MAC addresses if you do not want your ip address changing, ever. This would only be needed if you are forwarding protocols to specific machines through the firewall to an internal address from the Internet.
What model/make of wireless router are you using? Are you using third party firmware such as DD-WRT?