Someone could be using your computer power to mine bitcoins for themselves. Worth checking, it's been done before.
mining bitcoin doesn't overheat your router m8
inb4 article on router mining
I missed the thread that stated his router is overheating. That said, you have one or more of the following issues that may be eating up bandwidth:
1. Software updates (like windows, game or AV updates) taking place
2. Torrent server still active
3. Virus/bot running malware (like a mail server or ddos bot)
4. Possibly you wireless modem has a hijacker using your wifi (depends on type of wireless modem you have ( wifi router access or hotspot?)
5. Software backup/sync to cloud service running (Dropbox/one drive/Box/Gdrive/etc)
You have a little detective work ahead, but like mentioned earlier I would start with task manager to see what is active. Then try installing some tracking software and running your antivirus.