The credit card statement will contain a list of any amount carried over from the last statement plus any new transactions (new purchases and any charges such as interest/fees) together with details of payments made. This will give the new amount owing.
I would expect the interest to be shown separately from any PPI payment but maybe your lender rolls it all up me. Are you paying PPI- if so but it has never appeared on your statement then it would seem it's included in the interest charge. Without seeing your bills we can't be sure.
Have you asked your credit card provider for clarification?