When you go into settings - data usage- mobile data usage -what in there is using all The data I had the same problem since I am home a lot and on wifi there. One of the things I noticed was that the play store would update even when I had the data turned off and on wif. When I went into some of the settings I saw that most of these apps were allowed to use background data. While you are in that settings under mobile data usage just tap on any of the apps in there you will see that they can use all the data they want. I have turned off all of these apps but still a lot of them still use some data through the month. It has been a lot better but another one is the play store which I have set to do not auto-update apps. I am sure there will be a lot more suggestions for you on here like jrh4050 and ann154 have suggested.