Windows eating away at my internet

BigBadBeef

Admirable
I have a ZTE modem using 3G internet. My ISP has a weird system of fair use policy (excuse to charge extra money), once a certain traffic quota is achieved, the internet is capped to 384 kbit/s.

The thing is that this is more than enough for online gaming, youtube 360p streaming, media streaming etc... at least that used to be the case when I still had windows 7. Now I have windows 8.1 and something appears to be eating away at my internet connection. I already turned off windows update and all the apps (except the desktop button) but there is still something out there that is using up my internet.

By the way, I don't even have a metered connection button, and even if I did, the ZTE modem connection isn't registering as a metered connection so I can't even turn it on if I had the option.

Does anybody have any ideas on how to approach this?