Interest charges on paid up credit card account

I paid off my credit card account on the 29th of February 2016. This morning I received an sms with an interest charge on my account. Upon calling FNB credit card customer care, I spoke to Dawn Nzuza who told me that the interest charge was for a January transaction. <br> <br> The January statement had an interest charge of R310.49 & February R174.58. She said the charges were both for a withdrawal of R3,0000.00. How is it possible to be charged 16% interest for 1 month for 1 transaction when the statement clearly states that the interest charge is 23.25% per annum?<br> <br> I do not query the other account fees as i know those are calculated when the statement for the month is generated. I need rectification of this as I cannot pay interest on a paid up account and will not pay interest twice for a single transaction.<br> <br>