Request to investigate billing of data usage

I have a contract with Vodacom for two 3G smart tablets.I receive 500 MB and 500 night owl MB per month on one of the sim cards. I also receive 200 MB on the second sim card.I monitor my data usage often.On 9 October 2015 I had about 372 MB of data left. On the morning of 10 October 2015 I had 0MB of data as well as 500 MB of night owl data. On 1 November 2015 I receive 500 MB and 500 night owl MB.However,on 2 November 2015 I had 0 MB and 500 night owl MB.Meaning that in both the months of October and November the data 'disappeared' even though I did not use it.This is despite the fact that I switch off my data every time I finish using applications.System updates have been set to operate only when Wi-Fi is enabled.Auto check intervals for system updates have been set on manual.<br> <br> The total amount of data usage charged for October 2015 is R 1,577.47 and it is R 277.74 already for 1 November 2015 to 4 November 2015.However the data usage charged for September 2015 and August 2015 was R 275.05 and R 436.68 respectively,this prior to the incidents of the data 'disappearing'.My account has been soft locked since 5 November 2015&blocked since today.I now cannot send smses or make calls