Are there any other adverse effects that you may have noticed?
It has been reported that antidepressants can tinker with the body's thermoregulatory processes. Such things would naturally feel more pronounced during spells of hot weather.
To try to lower the temperature would be the usual measures, ie remain hydrated by drinking water, trying to stay in cool places, cooling the skin with say handtowels soaked in cold water and applied to the armpits or back of neck. Or even just spraying cool water.
Might be a good idea to take temperature readings and also as you have only just recently started taking sertraline, mention this to your doctor by telephone if you can when they open next week. (yeah I know, getting through can be a bit of bitch) Either that or discussing this with your dispensing pharmacist. Could be handy if they are on the ball and also have a good working relationship with your GP practice.