What ate my data?

[#1]

I have an iPhone SE. My package with Plusnet is 2.8mb monthly. I’m a light user usually never getting close to maximum usage. Except today

This morning I had 1.8mb or left to use before my renewal in 6 days time. 

At 1222 I got a text saying I’d used 80% of data followed shortly afterwards by another text saying I’d used it all. 

I left my house, and therefore Wi-Fi, at 1130.

How could I use that amount of data that quickly? No phone calls, no music, no browsing, no use of anything. 

[#2]

Hello @Davesona,

Thank you for getting in touch. I am sorry to hear your mobile data has ran out. So we can rule this out, did you have Wi-fi assist turned on for your device? If not please confirm your full address with postcode, full name, mobile sim number and account reference via PM so I can loo