Credit card interest

Please be aware of FNB credit card, I have been charged interest even though I pay the amount within 55 days, with the excuse I chose the minimum payment option which I was given when opening the account, and still was advised that it only charge after 55 days but its been charging me every month. Very unhappy about this experience considering I have 3 accounts with them. Thinking of closing all my accounts and going to another bank. Very disappointing.