Why is my car overheating?

I have a 97 Grand AM GT V6 auto. Over winter break, I noticed that my car was overheating, so I got it fixed. My cousin, who is a mechanic, put in a new thermostat and a new water pump because my heat wasn't working and my water pump was leaking. After he put all that in, my car was doing fine. The temperature gage was where it was suppose to be. Now that I'm back at school, my car is overheating again. At first I thought I just needed more coolant, so I put more in and everything was fine for about 2 days. Today, me and a couple of friends went to get some ice cream and we took my car. It overheated and died. Luckily, we were able to get it back to the school safely after driving super slow and putting more antifreeze in it. So do any of you know what might the problem be and how much it will to get it fixed? I don't know how to work on cars, and will have to take it in to a shop which I know is expensive. Thanks!