N300 WiFi Range Extender taking up all my bandwidth.

I called my isp to complain about slow internet speeds, they looked up my usage and all that jazz. Literally 99% of my bandwidth was being taken up by the extender uploading something. At the time i was closer to the router so I was connected directly to its signal, not the extender. So even with nothing connected to the extender at the time, it was still managing to hog all of the bandwitdh. ...... I change the password frequently, but again, there were no devices connected to it at the time, so its not an issue of others logging on and downloading or uploading or w.e., Any ideas on how to get this fixed? Yes malware scans have been done etc etc.