Problem downloading apps iPhone 3G

When I try to download an app from the App Store it asks for my Apple ID password.. I type it in then it says.."Sign In Required-Tap Continue and sign in to view billing information" (i click continue)It goes to the Billing Information, all my billing info is already typed in but it says invalid security code. I re-enter the security code but it contines to say invalid. Anyone know whats going on?All my billing info is entered correctly.