FNB Credit Card Interest - Dispute!!

Hi, I do not know where to start but let me make this short.

Basically I have a credit card which is 30 days interest free. I settle my account every month on the due date or the day before. When I check my statement, BOOM!!! I'm getting charged interest!!!!!

I have tried to contact Fnb only to be told that the interest is on transactions(how does that make sense when it is a 30 dayinterest free account)

I will be contacting FNB again to sort this matter however this is shocking!!

Please help!!