I had this problem before. It was either the phone losing the wifi connection or the home router disconnecting and the phone not reconnecting. The phone would often show it was connected to the router but just not using it for data transfers. If that's what happening, then I assume the data usage is apps updating or something similar? Make sure you have your phone to update only when you're on wifi. You can check you data usage setting see to see what app might be the culprit.
My high data usage stemmed from my kid waking up at 4 or 5am, taking my phone and watching youtube videos for an hour or two with the phone disconnected from wifi for whatever reason. It got so bad that I made a point of manually turning off data when I was at home just to be safe.