Just because you haven't been downloading or uploading, does not mean that your computer hasn't.
A lot of programs such as STEAM and world of warcraft will hide a downloader in the background, downloading patches, updates etc, often without you being personally aware of it. Programs which use a torrent for downloading patches (world of warcraft), will very very quickly eat away the tiny amoint of upload that you are allowed on the 10M package. There are others, so don't get hung up on the two Ive named.
My suggestion would be:
- Do a thorough antivirus scan with a freshly installed AV program or an online scanner.
- Check that your wifi is secure, change the password on it just to be double secure.
- Check for TSR's that might be downloading/uploading in the background.
- Install a bandwidth usage meter of some form on every one of your attached devices. Sadly the hubs don't have the capability to accurately monitor your usage. My Shub has a "meter", but its never accurate.
- Don't forget that it may not be you, or even your PC, but any device on your connection could be the culprit.