How to monitor and control data usage on iPhone?

I got an email saying I’d used 80% of my 30GB data usage before i started work. After work I got an email saying I’d used up all of it. I get regular breaks in my job and I am on my phone a lot in these breaks but not enough to warrant the remaining 20% of my data being used up in around 9 hours, what exactly is the cause of this? I’ve never had this happen before and surely background apps alone can’t use up 6GB of data in 9 hours? Is there a potential security issue with my phone? I’ve read before that hackers can run things on your phone which drains data

[Re-Titled by Moderator]