Excessive Data Usage (per Data Usage Meter)

Hello, over the past 7 days I have seen an enormous spike in Data Usage (per Cox's Data Usage Meter). I called Customer Service, they recommended that I reset my router's password, which I did yesterday. Whatever (or whomever) is piggy-backing onto my router, is also adversely affecting my ability to watch MLB.TV. I have a software program on my computer--Who Is On My Wifi--which has never shown any "rogue" device using my router. My computer has Norton Security (on 24/7/365), as well as Malwarebytes for occasional (at least weekly) scans for virus, Trojans, and malware--NOTHING found by either program. I used Norton's Power Eraser to double-check the effectiveness of the Norton Security--nothing. Junkware Removal Tool--nothing. Microsoft Malicious Software Removal Tool--nothing. I don't see a way to post a picture of my Data Usage Meter, so I'll pass on these stats: June 7-June 19: No more than ***12 GB per day***. (I watch MLB.TV on, basically, a daily basis.) June 20: Almost 150 GB! June 21: About 170 GB! June 22: 200.81 GB!!! June 23: About 155 GB! June 24: About 190 GB! June 25: About 145 GB! Obviously, SOMETHING is going on after June 19, and is STILL going on. Can't Cox track this issue down? I have 5 devices which use my router: HP TouchSmart 300-1020 desktop computer (wifi connected), Canon PIXMA MX340 printer, HTC One M7 cell phone, Sony XBR-55X850C Android HDTV (that's the device I use to watch MLB.TV, with an Ethernet cable), and Toshiba laptop (rarely used; but when it is, connect by secure wifi to the router). The router login is secured by a 9-character alpha-numeric password, with one special character. The pass phrases for 2.4GHz and 5GHz are also secured, with a 12-character password with upper/lower case, numerical, and special characters; using WPA2-PSK. Help, please! This is nuts! Thanks.