1995 Oldsmobile Cutlass Temperature too hot

I've noticed that since the weather is warmer, my car's temperature becomes warm quickly-within 15 minutes or less running the car. Last year, I told my mechanic and he installed a new radiator. My car temperature would always get hot during the warm months and I can't take it far or it is in danger of overheating. I thought the new radiator would fix the problem but it hasn't. I checked all my fluids and the oil and the antifreeze were a little low so I filled them. It's still doing it. I noticed a stain on the concrete where I park and it's on the driver's side. I tried to figure out what it could be but I don't know. What can I do to fix this problem or is it something major? I'd like to be able to use my car for longer than 15 min during the summer. I get so nervous that I will overheat in the middle of traffic!