A sweet smell when turning on your heater could be an indicator that the heater core has a small leak. The same coolant that cools your engine runs through small tubes in the heater core.
If one of these tubes starts leaking, you will smell coolant coming through the vents. You will want to stop by your service center to have this checked right away. When the leak is bad enough, the engine will over heat and create a big mess inside your car.