I thought your IP adress was assigned to your home address ..... I imagined it always stayed the same ..... is that not so ?
No, unless you have requested a static IP address, which most ISP's will charge extra for.
By far the majority of home users have dynamically assigned IP addresses. Each time your modem connects, your ISP selects a free address from its allocated pool of addresses and assigns that to you. That's then your IP address until such time as your modem disconnects from the network, at which point "your" IP address is then returned to the free list to be used again. The next time you connect, you're almost certain to get a different IP address.