Unfortunately you are not saying what exactly interest charge you paid and what was added. May be it was another interest for one of credit card services (or even for previous months) or may be some additional fee. In general you shouldn’t be charged twice the same interest (or fee) in one month.
Also all fees and interest rates should be clearly stated in terms and conditions to your credit card. If you do not understand something or it is not clear to you you’d better negotiate it with your credit card issuer.