Why is my internet usage so high?

[Mike]

What I would do is use a tool that can monitor and log internet usage, such as:
http://www.paessler.com/info/internet_usage_meter
See if there is any unusual activity. There could be someone who has hacked into your router and using the wifi. If there is no unusual activity and you find that the recorded amount is far less that what the internet provider is saying, you should take the record of your usage and talk with someone at the company and show them the proof that you are not using that much.