FNB scheming me

I have a smart spent loab with Fnb I saw they charged me 1000 for interest what is really the 1000 for as I have never skipped a payment mind you this a short team lon with no interest this is the second time I seeing this on the balance . I read my contract I signed with them they is nothing about interest that will be charged this is day light ******* I need this fixed as soon as possible tried to contact fnb but no one has answers I even went to the app to ask an agent guess what no one bothered to respond to the messages that I sent