iPhone x Data usage

Hello all, I'm hoping someone can help me with an issue that just popped up.

My iPhone all of a sudden is using an insane amount of data. I got a text from AT&T yesterday that I was at 75% usage for the cycle. I'm only 5 days into the new period.

I have 20GB per month and generally use 10 or less.

Looking through my previous statement usage numbers, It looks like end of last cycle large amounts of usage started occurring.

I haven't been doing anything different with my phone. I've had this phone for 3+ years. Same plan for years. My parents are on my plan, but they literally use nothing. It's all my phone. I don't ever stream movies/music. I use wifi most places I'm at. I get no signal in my house.

I'm at a loss what to look at. I called AT&T but they weren't really helpful.

Any idea what to look at? AT&T's suggestion was just to turn data off, but I'd like to know what's going on and why this is happening all of a sudden. This is happening when I'm not even using my phone. Like when I'm driving to work, or home from work(phone is not being used by me). It's happening when it's just sitting on my desk while I'm working.

The weird thing is, when I look at the data logs on AT&T's website it'll be several instances of like 450mb over an hour, or 350mb over an hour. All at times that I know I am not even using my phone.

Any ideas?

Thank you,

Marc