I am not downloading large media.
Your Internet Service Provider (ISP) thinks that you are, and they may be wrong. It is up to them to prove to you that the data use is yours.....with a detailed report stating exactly what was downloaded on what date from what site.
If they cannot do this, or are unwilling to do this, then I think that you will need to start looking for another service provider.
A few options if your ISP will not help you.
http://lifehacker.com/5917367/how-can-i-find-out-how-much-bandwidth-im-using-at- home