need a quick answer to a overheating question

[dellstopjeep]

so i have had great temperature with my engine all of its life. Never any problems with over heating and such. but then earlier tonight i parked at my friends house, left the car on and went inside, 15 minutes later the coolant resovior... cant spell it... smoking and the engine temp is around 230, i freaked out an just turned it off. this is an 89 cherokee so the coolant holder is in the left corner of the engine compartment, there is nothing leaking in the system and the resvoir is still full from where i last filled it. What caused my overheat problem?
also when i left my buddies house shortly aftert that and drove it had no overheating problems and stayed right around 210, which still is a little higher than usual but isnt a issue. I am goin on a big wheeling poker run ina week so i want to make sure this isnt a issue on the trail.
is this a thermostat problem or did it just get jammed and not let the system flow like usual?
thanks for the help