It sounds like your router decided to renew all the client ip address.
Yes, the last digit is the one which will change. 192168.1.x is a typical router DHCP table or 192.168.0.x The x can go from 1 to 255 and one of those will be the actual router itself, normally 1.
If the IP addresses change to often to be a pain, a way to stop this, is in the router, set devices to always use the same IP address.