That's normal. It changes whenever you receive bitcoins. The old address will still work if you want to continue using it.
Ohhh thats good to know
. Just noticed it change when I received my latest payment.Is there a finite number of addresses before an old one becomes unusable, or will my original address always work?
Bitcoin stores your addresses forever, so they will always work. Generating new ones is mostly to make it a bit more difficult for others to track your finances.