Chase Credit Card Problem

I signed up for a Freedom Card back in July 2011 which according to the invitation letter i got was 0% for one year. I used the card to buy airline tickets so was awarded the points for cashback. Ever since then they have tried to charge me every month interest fees, which they promptly refunded back to me until now. Today they are trying to tell me that i was not approved for the 0% interest, when in fact i had already faxed over to them a copy of the invitation i got when i was first approved. Now they are saying that it took them 3 months to check my credit which is excellent. And i have paid on time every month. I just hate paying interest fees.

What recourses do i have? Thanks for your opinions.