Are you absolutely sure about taking your torrents off as soon as they are done and that it is always uploading slower than you are downloading? With my torrent use, it is usually quite a bit faster going upstream than down. If you are using uTorrents, you can pull up "show statistics" under the help option in the menu and see how much you've uploaded and downloaded since the software was started. I haven't dug around to find it, but I think you can reset the statistics under options and monitor the torrent usage from this point on. I presume similar functions exist with other torrent clients.
In theory it is possible that someone has installed something nasty on your computer via hacking or malware, and is using your computer as a distribution point for files, but this isn't very likely, IMHO. I'd take a good look at your firewall to make sure it is totally locked down except from the applications you've enabled to burrow through the firewall. I'd also look at your firewall to see what logging options you might have and monitor what traffic is outgoing, and from what software.
My experience with the Bell unlimited/non-unlimited account is - at least for me - they cost about the same. I just think of my account as being $75/month because I hit my max usage billing caps each and every month, without fail, to the tune of about 200gb a month.