IPhone 4 with Verizon....?

I've downloaded a few FREE apps from the app store and a couple days later when I went to download something else it kept saying my pin code for my card was invalided but it was fine a dew days ago. Now it won't allow me to download any apps because it keeps prompting me to enter the security code (that 3 digit number on the back of the card) and I've tried several times..... It's just annoying