Discrepancies on billing

My contract is a 2GB day data with 2GB night data.<br> The invoice received Jan 2016 of R1628.47 has some discrepancies. <br> The total data used as per the Itemised billing, this calculated by myself is Day - 2,01GB and Night - 1,65GB.<br> Itemised billing invoice shows that at exactly 01:09:47 I used ***KB data this costing me R1056.38, yet the total night data used was below 2GB. How on earth is that possible?<br> The Huawei modem I received with contract allows me to log into 192.168.1.1 to connect to internet and also notifies me when my data runs out.<br> The notification of data running out was received at 01:58:14AM yet the data I'm charged for was within 2GB that I'm entitled to (firstly) and that data I'm billed for was used at 01:09:47AM (secondly). How faintly possible that I'm charged for data within my allocated time of using it? Can you please investigate this as it clear as day light that I'm being charged for what I originally signed up to receive?