Sudden excessive data usage

I had a 4S for 2 years, & now a 6 for 8 months. I never even came remotely close to using 1G of data. But 3 weeks ago I was notified by Verizon that I was close to my limit, & yesterday that I was over my limit, with the primary usage being games. I do not play games (nor download videos). In fact I don't have even one game app. I've been to the Apple store, where they emptied my phone and had me start from scratch. I've been to the Verizon store, where they did something to my network connection. I've been on the phone with Verizon twice. No change. The data usage keeps rapidly mounting. I have wi-fi at home, and in my office. I have added no new apps in the recent past.

Any ideas as to what could be causing this change/increase in usage, &/or what to do about it?

Thanks in advance.