Sudden data usage spike!

I've received the notification of data overage in Feb. But I didn't add any devices and only watched streaming videos at night. We usually only use 60%-80% of our data plan. I want to know what went wrong!
The weird thing is that I received the first notification saying that I had used 90% of my data plan two days before the second notification of 100% usage. 
No way we would deplete 10% of the data plan in just two days! We only watch streaming videos at night for 6-7 hrs and we don't download any videos. Our data plan should allow us to watch HD streaming for 16 hrs per day.