Is someone causing me to go over my internet cap?

I have a cap on my internet connection 300gb and every month it goes over my limit to 400gb. I log all my data on the computer and it uses less than 300gb. What could cause this increase in usage? Is someone sending me incoming data? I am connected to a modem directly with no router, and I have disabled firewall (for my needs I need to disable it). Could this be the cause? How can I tell if I'm receiving an incoming request? My IP hasn't changed for years, and I don't have any application that regularly listens to any ports