2003 Hyundai Elantra my 2003 Elantra is over heating!

My car is overheating, after about 15 mins of driving I notice the needle going up to the mid way mark, now usual my needle stays at about a quarter of an inch below that, if I turn the heater on all the way high, it cools right off. Today I had to do a bit more driving then normal, and its 72 degrees here in virginia beach, and even with the heater on fulll blast, the needle raise to be higher then the middle hatch mark, and when I slowed down it dropped down to the almost normal spot, oh and also if I am at a full stop the needle goes back to its normal position under the halfway mark also its a 2003 hyundai elantra gls