iPhone 4s personal hotspot Sprint

This morning my iPhone 4s had the personal hotspot option showing in my settings between the notifications and location services. When I tapped the arrow it gave me a password and option to where I can hook it up via wi fi Bluetooth or USB. I chose Bluetooth and tested my iPad and it paired with it. I had my 3G from my iPhone paired with my iPad. However I'm not subscribed to personal hotspot with sprint...and after a minute or two it stopped. There was no more personal hotspot. Just wondering why this might have happened.