Check the thermostat to make sure its not worn out or damaged. Just pull of the left hand side fairing (the side of your clutch lever) and it will be at front of the radiator at the bottom. Just pull it out and have a look.

My CBR250 also heats up if im stuck in traffic and not moving. Its just cause no air is flowing through. The fan should kick in around 3/4 of the way, if your not sure its working just get off the bike and have a look, you would be able to see it.