I don't know if you are using a laptop on a wifi signal ?,
if so it might be that windows is using your bandwith to broadcast updates of itself to the rest of the world.
Windows 10 does this by default , it's also sharing a lot of info to Microsoft.
Lucky for all of us it can be turned off
Here is how to turn off the update sharing for a start
http://thenextweb.com/microsoft/2015/07/30/windows-10-steals-your-bandwidth-to-send-other-people-updates/
i would suggest to use google and search on windows 10 privacy tips to find out more of what i shares by default and if you want it or not.