See if your ISP has a day-to-day calculator thing for your usage.
If so, check what day it spiked or if it's a constant.
If it spiked, talk to everyone in your family about hwat they did that day on the computer.
If none of them did anything intensive, (torrenting, downloading large files, downloading suspiscious things) then check your router page for recently connected devices and see if there's anything on there that's odd or isn't from your family. If so, change hte password and call your ISP to tell them your bandwidth was being stolen.
If there isn't anyhthing odd, about any of it, it's likely you have some malware. Perform an intense/deep scan for malware, adware, viruses etc.