Its called a Dynamic IP.
Basically, Your given a "lease" (Which Cromewell Spoke about) And it sounds as tho your ISP is only allowing you too have a Lease for a period of time.
I too am curious why you would be getting New IP's Why your actually Using the internet.
You might experience a bit of a lagg while the Renewing Process is happening.
Changing IP's is not a bad thing so don't worry yourself.
With Local Area Networking, I have my server setup so it will give out a new IP every 7 Days too all my PC's
If i wanted a IP Earlier, I could use Command Prompt Using:
IpConfig
ipRelease
ipRenew
And that would renew my IP.
Kind of Pointless, But good too know.