Data disappearing

I am so disappointed in Vodacom. On Friday 1 September 2017 I loaded my account with 5GB. On Sunday 3 September 2017 I got a notification that my data has run out. I mean, how can that be. I have only used it for checking my emails, whatsapp messages and facebook. How is it possible that my data ran out. I did not even update any of apps and my settings are not on automatic update either. When I check "my bill so far" it indicated that my account is currently standing on R1200. This amount tripled because I have a top-up package that comes to R410 monthly. On their statement it indicate that I have opted for additional 5GB extra which I was billed for-for the month of August and September.