UNACCOUNTED DATA USAGE

I noticed a pattern for the past 3months regarding my data.I regularly check balances,as I don't want to go over my data to avoid unnecessary charges.I noticed a positive data balance of +- 200mb left at 20th of each month,and then when I check my balance again I would have less than 20mb left,which is impossible,I have wifi at home and work,therefore uses minimal cellular data.On 20/7/15 I received a sms at 05:05 notifying me that my data bundle has depleted.The night before I checked my data balance,and I had about +- 180mb left. After receiving the sms,I checked my balances,I have incurred an extra bill of +- R140.00.I emailed Cell C's customer care, and received a case number.I received a phone call from a consultant on 22/7/15 to clarify my query.She advised that a data validation report would be sent to me in order to view my activity and data usage,and this will only take two-three hours from end of call.I NEVER received this email.I emailed Cell C on 29/7/15 asking to provide me with the report,they once again FAILED to respond.Cell C clearly disregard their customers.Cell C not only has poor network coverage but also POOR CUSTOMER SERVICE!!I will NOT renew my contract!!!!!