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
http://support.apple.com/kb/HT4098
iTunes Store: Changing Account Information
http://support.apple.com/kb/HT1918
iTunes store support
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