your modem gets an IP address from your ISP which is good for some amount of time (3 days is a common value). This is called a "lease"
At the end of the "lease" your modem will release he IP back to your ISP and ask for a new one. Most of the time its probably the same IP, but it CAN change.
It's called a Dynamic IP address, Which means that your ISP can change the IP address any time your current IP address lease expires or when you reset your modem. I suggest you Google "Dynamic IP address" for more information.