Security code problems. help!!!

Everytime I try to update and app or get a new free one, iTunes asked me to sign in to update my billing information, and it's telling me that the security code I have entered is an invalid code. I've entered the 3 digit number on the back of my credit card many times and it's still showing as invalid, my credit card doesnt expire until 2015 and has money on it, I just dont know how to fix this problem!!