At this point, you will need to resolve this with Apple/iTunes Support.
You can look here for contact phone numbers.
Or you can email iTunes Store support by clicking on the link under the iTunes Store support section and then click on Billing, Purchases and Redemption in the next window.
What is the primary issue that led you change pw and billing details? If being billed and you don't know why its most likely due to an 'in app purchase' that you are unware of. This is, an app you downloaded that bills your froom within the app such as a magazine/newspaper subscription.
You can turn off in app purchases on your ipad. Go to settings/general/restrictions/ in app purchasees-turn off
iTunes Store: Purchasing and managing auto-renewing subscriptions
iTunes Store: Changing Account Information
iTunes store support