My data usage doubled from previous months.

For no apparent reason my data usage doubled past month, causing me go over the data cap. I was charged $60 on top of my bill. I reviewed all connected devices on my router, didn't see any unauthorized device. Changed my router password. 

Today I checked my data usage on the Xfinity app, to my surprise I've supposedly used 587GB in just first 7 days of July. I called customer service if there is any error on Xfinity data usage meter. I was told there is no error on Xfinity data usage meter and I should pay for unlimited data for $30 since I use my own modem. 

I am so confused, how my data usage doubled suddenly. I have the same devices and no change in data usage habit. Has anyone else experienced this??