If you have trouble maintaining your regular pace, slow down a bit. And if you become light-headed, slow down and walk for a while. If you experience chills, stop immediately, and find a shady place to sit down. You must allow your body to cool. Even though you're having chills, you're not cold.
Some people have more difficulty dealing with the heat than others do.
Take it easy during the hot weather, especially if you've had trouble dealing with heat in the past. Once you become heat stressed, you are psycologically susceptible to it for the rest of your life. Run at sunrise or sunset. By running in the morning you avoid hot sunlight but usually face more humidity. By running in the afternoon you avoid high humidity but must brave the sun.