why my iphone use high quantity data?

My iPhone was using 10GB per day until resettingto factory settings. For 5 days I exceed all my limits both 3G mobile operator and home wifi fiber internet operator.

Does any body have any idea what may the reason for that? Before the problem occurs I've purchased speedtest.net application. Can one application use 10GB data a day? If it can, is it possible to do so even if I close all the applications by double clicking home button. I'm scared and can not any applications except some reliable and most used apps.