The most common would be checking for unauthorized usage of your wifi router if you have a wireless services at your location.
Look at its logs and usage charts to see if there is heavy use. Change the wifi password and see if usage drops.
Do you have an older media player or devices like an early version of a DirectTV receiver?
Something we’ve seen several times is the older DirectTV receivers prompting you to connect them to the Internet. If you pick a series you’d like to watch, the device downloads everything it can do your receiver, making it a highly inefficient and data gobbling, ‘on demand’ streaming device.
Check other Internet connected devices that could be downloading a lot of media, using up a lot of your data plan.
Failing to find the cause with the above methods, disconnect everything you can from your router and wireless service. Log into your router and keep an eye on usage as you add devices back on until you find the culprit.