Incorrect Data Usage Displayed

I have received consistent monthly messages from Cox stating that I am going over my data cap. I pay for the top internet plan with 400gb of data. How am I going over this? I work full time and am only home a few hours out of the day to watch Netflix - which, is maybe a 30 min to 1 hour movie a day. I rarely use my computer and do not leave it on when I am not using it. I do not have a virus and my wireless network is secured. The only reason I have increased my internet plan is because I have received these messages from Cox stating that I was going over my monthly cap. There is really no reason for a user like myself to be on the ultimate plan and I do not know how I am using 15-35gb a day. That just doesn't make sense. It seems to me that the Cox data usage meter is way off.