Another option I forgot to mention is you may in fact be operating in Ultrapeer mode. If you open the Connections window, see how many people you are connected to. If it's more than 5 then you are in ultrapeer mode which is why you're receiving extra traffic. This can be disabled under Tools>Options>Speed. I believe UP mode can add anywhere up to 20% load.

Yes you may need to keep a check on your usage over a day or week to see if it will potentially go over your monthly allowance. If yes, then reduce upload slots or bandwidth.