Looking back at your previous post, I see the word 'upgrading', which I take to mean that the operating system on your iPhone has updated itself.
Such updates tend to use very large amounts of data. (As an analogy, there have been lots of posts here from people who've found that upgrading to Windows 10 on their computers has used up nearly all of their monthly allowance on capped broadband services).
So it's likely that it's that 'upgrade' that used nearly all of your data allowance.