If you have Windows 10 (I *think* 8/8.1 allow it too) you can sort processes in your task manager by network usage:
Open Task Manager
Go to "Performance" Tab
Click on the "Network" column heading
Every second the list will sort itself with the most network intensive processes at the top.
When you've identified which process or processes are gobbling your bandwidth, see if you can figure out what it is and what it's doing. It might be legit (Steam downloading game updates for example) or it might be malware up to no good. Malware will probably hide itself an otherwise un-suspicious windows service, so it's sometimes harder to naildown. But find your offending process is step 1.