Well your public IP does change...The external IP/Public IP is the one through which one gets connected to the internet and it is classified into static and dynamic where static IP does not tend to change and the dynamic IP varies each time the modem is switched ON and OFF.If you have a dynamic IP then there are chances every-time you log on to the internet you are assigned a new IP.The duration to the change in IP depends on your ISP.Also if you vary your type of internet connectionthe Ip changes.You can look for the change in IP through Ip-details.com and confirm it.