DELAYED BILLING - LATE NOTIFYING OF CUSTOMER

My 2 year contract has come to an end. I was billed at the end of March for the minutes and the extra data i added to my account for the previous month. I have just been billed for the month of April as well as March for the data used on my account. I cannot understand why I wasn't billed at the end of March for data used in March and only in April.. My bill for this month is doubled up and i cancelled my contract as I have never been completely happy with the services from Telkom.. According to a Telkom agent my contract was cancelled the 7 March 2016, When i was in the store on the 5 March 2016 to cancel it. I understand that it usually takes 30 days to be cancelled so i will patiently wait for the 7 April to become a prepaid customer. I need an explanation as to why I am being billed in April for March data?? I have been on hold for the last 27 minutes waiting for an answer. I am utterly DISGUSTED IN TELKOMS SERVICE and im sorry but i will never ever recommend Telkom to anyone i know.