Iphone 6 plus use to much data

I Just purchased the iPhone 6plus 5 days ago, and I received a message from Verizon saying Ive already used 90% of my data alloted for the month. Now I know I haven't used that much data cause I haven't even used my phone hardly, the cellular data has been turned off and all apps I do download I use wifi connections. I took it back up to the verizon store and they couldn't even figure it out. They even went through my phone to see the activity and nothing turned up to cause such data usage. Has anyone been experiencing this problem? If so how Was the problem fixed?