High internet data usage for no reason?

[Bita]

For the past few days, I have seen my internet usage go through the roof. I see downloads of 70-80gb per day. I have changed my modem and reinstalled everything with new logins and passwords, yet still the issue remains. There is no one else home but me and I am home only in the evening so there is no possibility of download of 80gb!
Can anyone give me a clue as to how this may have happened and if there is a solution, then where can I look within my pc to find out?
I have an antivirus and have scanned my system 5-6 times already and every time nothing was found. I also don't torrent and only watch youtube. So I don't see where this could originate from. My cable providers is also unsure.
Please advise.

[Joe]

For work

[Harley Drive]

you either have a virus/malware or some one is stealing your connection, change the router password and the wifi password

[Savy Bones]

learn how to change your password and change it weekly,
or go further and exclude all MAC addresses but your computers. sounds like someone is cracking your wifi, and if you think NO WAY, yes it takes about an hour or so unless you do something to stop it from happening.

[PokerPlayer]

Are you sure you aren't streaming music or videos on your phone or something connected to the internet in your home.