Why did my iPhone data deplete after connecting to iTunes?

I updated my phone to 17.4.1 last week. I connected it today to my Windows 10 laptop to load from ITunes. iTunes said it needed to update my phone to 17.4.1 - strange? I left the phone connected to the laptop and twenty minutes later my phone bleeped and I had a message to say that I was almost out of data! I only use about 2 GB a month and now I have .75 left. What happened?

[Re-Titled by Moderator]