You've probably thought of this but go to your normal data usage in your settings.
Look for an app that has a large usage (assuming you've used alot of your hotspot data) & if it has a name that looks related to hotspot there you go.
Cause on my phone at least if I go to data usage it'll show an icon & label saying: mobile tethering & hotspot , in the list with all my other apps & it being the one with most usage