I'd check the data usage directly on your phone to see what used so much. It could've been a software or app update, or something was accidentally left streaming perhaps?
iPhone: Open Settings and tap on Cellular. Scroll down and you will see a list of apps, with the data usage for each app beneath each one.
Android: Open Settings and tap on Data Usage (it may be listed under Connections or Mobile Networks depending on the manufacturer of your device). From there you can see a list of apps and their data usage for the set period.
You can also go back up to 90 days in Self Serve to see where the usage took place - just change the drop down from "Last 7 days" to "Last 90 days", click View Usage History, and then scroll through. For that much data usage I would imagine that a block of data should show up.
Edit: I just saw the reply in regards to it being related to mobile hotspot usage. Check your settings, depending on your phone you should be able to limit the amount of time the hotspot remains available and also control which devices can connect to it based on the MAC address. By the sounds of it, your computer may have downloaded an update of some sort which would've chewed through your data quite quickly.
If you're using a Windows 10 PC, you can limit the amount of data used in the background in addition to restricting Windows Updates to prevent high data consumption. To do this, open Settings > Network & Internet > Wi-Fi > Manage Known Networks > Properties and toggle Set as metered connection to On.
