ITunes says my card security code is invalid?

Ok so I have a visa gift card that I got for Christmas and i bought some music and few apps on it,which used up the balance of the card,so obviously I know I can't buy apps or anything but now all of a sudden iTunes won't let me buy any more of the free apps,and won't let me download the free updates for the free apps I already have,saying that my security code is invalid.I typed it in a million times and I KNOW I'm typing it in right,but it just keeps saying invalid.so what's the problem,is it because my balance is used up?and if so what can I do about it?I'm only 16 I don't have a credit card and my mom won't let me use hers even when I promise I won't buy anything but the free stuff.