Monthly Data Allowance Disappeared

[Magictorch]

My 12 GB data plan renewed yesterday. Received message at 5.00 pm to tell me that I had used my allowance. Checked my phone and my sim was showing no data left. I used my phone normally that day - no streaming of media. I am certain that I did not use that amount of data in less than 1 day. 
This must be a technical glitch but how do I prove it? 

[PandJ2020]

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@Magictorch wrote:

This must be a technical glitch but how do I prove it? 

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You can only but call Sky on 150 and see what they say.

Check your app data settings on your phone as perhaps a large update was downloaded in the background?  Data counters may provide a clue.