pressure in gas tank

Got a question for whoever can solve it.
Now from what I here there is supposed to be some pressure in the tank
Great and dandy if so but how much is enough?
Opening my gas cap after driving for 15-20 min sounded like one of those pressurized cans of air.
It looked as if my purge canister wanst working becuase so much mist came out it seemed as if I were looking through a fire with everything being so wavy.
I did change my gas cap but that changed nonthing.
If anyone has an idea please notify me ASAP.
Also is there any chance that this could lead to lower gas mileage.

you could have a plugged vent line or your fual pressure regulator is stuck closed. But it is most like your evap system pull the line of the tank and canister and blow threw it should be free if that is ok try runnung it with out the cannister hooked up if the problem goes away replace cannister if fuel comes out of it check fuel pressure regulator and for a block return line. Good luck