iTunes - Billing Info - Invalid Security Code

I'm new here, but I think this is the right place to post this question...

I have used the same billing info on my itunes account for over a year, but within the past couple months it has decided that my "Security Code" is invalid on my card. It is not... I've checked and re-checked a thousand times. My billing info is still the same, and used to work just fine... but now it won't accept my security code as valid for no apparent reason.

Help?