Why does my iphone 4s use so much data?

I got this iphone 4s a little over a year ago, and before the iphone i had an android and i never went over my data not even close, each month i get 2gb of data and every month i have gone over once or twice and recently it has been more like 4-5 times i go over a month. The weird part about it is that I factory reset my phone, I do not have any apps on my phone other then the ones that come on the phone, and i only use my phone to make calls or text, just to see if that would stop the problem because it is getting very expensive. And yet even with nothing on my phone I STILL go over the data about 5 times. I called att and they connected me with apple and i was on the phone with them for about 2 hours trying to figure out why they didn't know either. They said it was using a lot of data but they didn't know where it was coming from because i don't have any apps on my phone, i haven't been using the internet, or fb, or anything like that stricktly calls and texts thats it. Does anybody know why this could be because apple wasn't a whole lot of help?

You most likely need to be turning off your apps (even the ones that came with the phone) because they run in the background. Also, turn off bluetooth and location services while not using them. Don't stream anything. I would take it into an apple store if nothing helps and see if they can either figure out the problem or replace the phone for you.

Becasue you might have enable lots of background apps like whatsapp, fb, twitter and these suck data