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.
Disclaimer: This editorial content is not provided or commissioned by the credit card issuer(s). Opinions expressed here are the author's alone, not those of the credit card issuer(s), and have not been reviewed, approved or otherwise endorsed by the credit card issuer(s). Reasonable efforts are made to present accurate information, however all information is presented without warranty. Consult a card's issuing bank for the terms & conditions.
All rates and fees, and other terms and conditions of the products mentioned in this article/post are actual as of the last update date but are subject to change. See the current products' Terms & Conditions on the issuing banks' websites.