Why is my A/C is causing my car to heat up???
 

I have a 97 ford explorer XLT. The other day I drove it to the mall which is about 10 miles away from my home. I got in my car and turned on the a/c no problem going to the mall, but when I was comming back and I was stuck at red light I noticed the temp starting to go up slowly but still on its way up. The light turned green and i hit the gas, once i did that I noticed the temp started going down. I stoped at the gas station and did not turn off the engine left it running with the a/c on just to see if it would happen again, sure enough the temp started going up, it passed the thermostat icon so It was way above the middle mark. bottom line is while the car is in drive and the a/c is on it won't warm up but if you come to a stop and a/c is on, the temp starts rising. I now drive with my a/c off and the temp stays in the middle, I do notice that it tends to go down a bit when I'm driving and when I come to a stop it goes back up to the middle but it will never pass the middle mark. (as long as the a/c is not on) If anyone has any idea of what the problem might be I would really really appreciate some feedback. :)