I've used a Jetpack for years at home for internet. In July I went from 5gb per month to 28gb+ and am now over my 30gb limit. Nothing has changed, same computer, same daily activities.
How can my monthly data usage change that dramatically? Again, I have changed no hardware or activities. I use Facebook, email, and play an online game and have for several years now. I see no evidence of an unauthorized device connecting to the Jetpack.
It is important that you are able to manage your data as you see fit. Has your computer been updated recently? Is the network and sharing center configured to use a metered connection?