A few months ago I started having this problem.
I have a 2gb data allowance. I bought an add on of 4gb. For a total of 6gb of data a month. The problem is that at the beginning of each month it started saying on the app that I'd used nearly 4gb of data when I've still got 30 days left.
I've not done anything that would use that much data, I've deleted apps that could be the cause, I've turned off auto updates, I've got data saver turned on,
It only happens at the beginning of each month, if I turn data on throughout the month it gets used up at a steady pace, 50/100mb per day.
Please if anyone has any info that would be great, I really feel screwed over.
And to repeat, I don't do anything that would use so much data, watch movies/videos for example. There must be something wrong on their end.