Yup...It can be the coals. Pleh that is why I don't like QLs too much. Some give off less chemically sort of taste compared to others. But, they taste kinda nasty when you smoke with them after a while.
Also...Your headache...Did ya make sure your coals were RED HOT and ashed overbefore you put them on the bowl? Perhaps that might help ya out
Orrrr maybe the coals just gives off more chemicals and Carbon Monoxide
Also, make sure you drink or eat something b4 or during smoking or else you'll get a headache. Pleh that is one of the downsides with QLs