Data usage of R3000.00 in 1 day

I'm receiving a message every morning to inform me of what my usages / account is thus far.<br> From 4 May 2015 to 5 May 2015 my account increased with R3000.00.<br> I immediately phoned customer care so that they can have a look and see what the problem is, because clearly it is impossible... One incompetend call agent after the other could not assist... I asked the one guy to open a query and investigate, as there must be something wrong. He helped me to check if all the settings on my phone is correct and confirmed that everything that's suppose to be switched off, is switched off.<br> On 12 May I got a call from the investigator\who informed me that I'm using that much data because of an app that's asking a user name and password