Most carriers disable hotspot capability unless you pay them an extra monthly fee.
If you root your phone, you can usually bypass this and enable the hotspot. (I mention that because it makes no sense for a carrier to both restrict hotspot and give you a monthly data cap. You paid for x GB of data. It shouldn't matter to the carrier if you use it with your phone or your computer. It's like buying a meal at a restaurant, and the restaurant trying to charge you extra money because your kids want to eat it instead of you.)