incorrect data bundle billing

I am currently on a Smart L contract and get 1Gig per month with the contract. I have a Samsung S5 and usually get around with the 1Gig. For some reason I ran out of data on the 24th of March and also received a sms from Vodacom informing me of my bundle. I purchased another 1Gig data bundle knowing it would be more than sufficient. I received my bill this morning and to my utter disgust I apparently used 2912.62 gig for the month and that is impossible. I do not have automatic update switch on as I do this over WiFi. I contacted Vodacom to log a query only to be told that they do see that I have used the data, I ran out of data reloaded on the 24th and ran out of my 1Gig again on the 28th of March - that is impossible as I check my data each and every day - I always get through the month with my data - how is this possible? I've been told that I run updates after 12 in the evenings that's also untrue. No one can help me with my problem and find this shocking as I ran out of my usual 1 Gig purchased another 1 Gig and getting a bill for a further R368.91 for my data?? I have been told to manage my usage, but there are no updates done on my phone without WiFi. How can data disappear?