Overbilling on overdue accounts

In October 2017 I requested an amount I owe on my account, they said R2600. Last week their lawyers contacted offering a settlement account, I agreed and they said they will send me the settlement amount via email. They emailed me +15 000 that I need to pay before end of December. How come my account went from R2600 to R15000? My account credit was R2000 ,last payment was made in earlier this year. How do you charge interest per month for my account to rise that much