FINCHOICE IS RIPPING ME OFF!!!

I have a loan with finchoice and I'm paying monthly installments. All was well until I decided to pay an extra R2000 so that I could pay off the loan quicker.I then noticed on my app that the loan balance was increasing which I do not understand because in my understand the balance should decrease because I paid more not increase .I gave them a call and the agent that I spoke to told me that I was paying interest but she couldn't explain where the interest was coming from .Why should I pay more interest just because I decided to pay more than my monthly installments?plz make it make sense?