When i start the car with out the heat on the water temp goes to normal, which is a little under the middle. But if i let the car warm up all the way and then use the heater for a while it will drop to around the bottom of the gauge, righ near the first line. Any ideas why this would happen?

 

probably because by utilizing your heater, youre providing more surface area for the water to contact, thus relieving itself of more heat thats a good thing, thats normal advice, if you're in the desert and starting to overheat, turn off the ac and turn on the heater to help out your engine