Why is my internet bill so high? Internet problems? HELP.?

I have a BT HomeHub on 10 GB a month for �10. Since I switched from dial up to wireless, my bill has been almost 3 times what it used to be, for unknown reasons. Looking into it for 10 GB people have said you should be able to download up to 32 HD full length movies per month or up to 5000 albums, this however seems to have been done in the first few days. We don't have many things connected to the router, the main being a PS3, Macbook and an iPod touch. None of which we download things from constantly. Recently there have been updates on the PS3 but they aren't big enough to fill 10 GB of internet, and the iPod is rarely used too. The other it the Macbook, which is only used for browsing twitter facebook, blogging and the odd video a day on youtube, again this can't be the cause of nearly a �30 bill within the first 9-10 days. The only thing that has come to mind that our big computer has been 'running' or downloading so to speak constantly and wont stop the whole time that we are on it, we think that the cause of this is that a person or multiple people have hacked the system and are using our internet and downloading constantly without us knowing that any of this is going on, however something is going on somewhere along the line because using the device for a mere 5 hours a day just on social networking sites could not possibly bring the bill to �30, this isn't the equivalent to 32HD movies surely. Does anyone have any suggestions as to why the bill is so high or how to resolve this issue? Many thanks.