IPod Touch App Store "Invalid Security Code"?

I have a $25 prepaid vanilla mastercard that I have used to buy paid apps on my iPod touch, then one day I tried downloading a "FREE" app and it asks for me to enter my card's information for some reason. So I wrote all the information but it keeps telling me that my "security code is invalid" even though it is correct. I can't download "ANY" apps anymore! How do I fix this? Please help. Thank you.