Have you installed any new software lately? Any new devices in the household?
Well if you are running a Windows based PC / laptop, you could install Networx. It will only give you the detailed information for that specific PC but it may help. It's also FREE. I've got it on my PC. It also gives a breakdown of which apps are using data too.
Any Android devices you have, you can go under Settings and see the wifi data usage and breakdown per app.
If you use any wireless devices, ensure that the router encryption type is WPA2 and that your password is complex (ie not a normal word or string of numbers but a mixture including uppercase and special characters like !@#$%^&)
Outside of that, a complete malware / antivirus scan couldn't hurt.