What do you mean credit card format? They are still the same size as before, same info on them..
Sounds like you mean a web page with lists of cards ie from a banks website or comparison website - that is likely to change if the web page administrator wants to. You stored an address in your browser, and the website owner changed what that address contains. The bank could have decided to ask you for information so they can contact you and sell you a card.. and thats the address your browser looks at
If you saved a secure web address, ie a credit card statement, then you would get the message about logging in.
Really sounds like the web page you saved has been updated by the website owner.