DATA Disappearing

Using modem ***) on a 2Gig subscription with Vodacom. Data has been disappearing at a rapid rate. But only yesterday did I notice a lump sum of 600MB disappearing in an instant! Called Vodacom, escalated the query and today they responded that the Data was under normal usage and the 600MB was consumed at 9pm on Tuesday. I only noticed Wednesday morning. Now we had load shedding in my area (Olievenhoutbosch) on Tues from 8:05pm till 9:05pm. I remember correctly that when the lights came back on, I prepared food, watched some TV and only started working on the computer after 10pm. So this story does NOT add up. Vodacom is ripping me off on this data deal. I want to cancel this contract as it has been problematic this year.