|So it's most likely, that one of your computers is infected by a virus or trojan.|
Some trojans or viruses do upload things in background and the user did not recognize it.
I suggest, to start only one computer at a time and watch the routers LEDs for activity, while you do nothing on the computer.
As mentioned, do this one by one and you should be able, to figure out, which computer causes the upload.
When found the culprit, use taskmanager to figure out, which process the highest cpu utilization. If you can't figure the process, disconnecting the computer from the internet might help, cause such trojans or viruses will try harder to connect to the internet, which causes higher cpu utilization.