When I first log into the iTunes store it shows me next to my log in name how much credit I have left on my gift card. Once I've bought a song, it disappears and can't seem to get it back. I definitely haven't spent it all?
I think you should check how much did you spend and if it's everything correct, because you have no credit left... If you want to want to report it to Apple, just click [here|http://www.apple.com/support/itunes/store/browser> and fill out the form!