Did you create it using the "Hotspot" app that came preinstalled on the phone, or using a third party app?
Do you have a notification in the bar at the top of the screen that it's still running?
You'll only be charged for data if you use data, so if it's still on, but your computer/other devices are connected to your home network instead of the hotspot that you created, you should only be charged for the data that you use on the phone itself (unless you've got the phone hooked up to your home network via wifi as well).