Account Handed over

Yesterday Woolworth called me and told me I was behind on payment, which I completely forgot about making payment. I made payment arrangement for yesterday R320.00 but only paid the money today. Later on today I receive a call from ARA Group telling me that my account has been handed over and I need to pay R470.00 on this account. No letter of demand has been received or calls beside the one I received yesterday. How does an account accumulate over a R100.00 worth of interest in one day.

Please