Charged More interest than I am supposed to

I have a lone with capitec. On a monthly basis I pay extra into the loan. During March my interest in the loan was R1300 for April the interest on the loan is R1328. How is that possible especially because the interest should decrease when the Capital amount decreases. Why is it that the interest rate increased. To top it off I call your consultant and after realizing that I am in actual fact telling the truth she put the phone down on me. That's disgusting.