MTN Excessive Billing

The first thing that I did when taking out a new Samsung A32 cellphone contract, on the 17th of June 2021, was to set the usage limit to R0.00. I have the contract to prove this. I have used this phone for more than a year without issues. I load data and airtime as needed. This month my account amounted to R4173.00. And when I phoned in they said that I still owe R7000.00. I am not sure how the usage limit can all of a sudden increase without me knowing?