Q: I have problem purchasing songs on itunes with iphone 4s

I have redeemed a itunes gift card for $15 on my iphone. When trying to download 2 songs it said my credit card was invalid. I needed to update my credit card. I entered a new one and all info as soon as I entered it itunes asked if i would like to continue to download the songs at which time I said yes. the songs downloaded and my itunes account says I have $12.25 credit. It billed my credit card and my itunes gift card. what do I do can someone help me?