Large amount of data used in a short amount of time

I kept getting messages that my mobile data was almost finished but I wasn't using my phone. So I continued to switch off all apps on my phone and only left WhatsApp running. (Keep in mind my phone is set to do all backups and updates in wifi ONLY) I purchased another 500MB as I received a SMS saying that my data had been depleted! In that time I drove home which takes me 20mins. Within that 20mins 400MB was used and I was not on my phone and all other cellular data switched off. I phoned vodacom to complain or atleast find out what was going on and they blamed it on the type of phone I had. So I said but I have had this phone for almost a full year and nothing like this has happened before. So they continued to tell me that I must at all times switch my mobile cellular data off. What is the point of having a phone then? I am very disappointed in Vodacom!