The most likely reason is that you have too much heat on your hookah bowl. Remove some charcoal and if it is still harsh then that means you burnt the tobacco and will have to re-pack a new bowl.
Some people will knock off all coal from their hookah bowl, then take the bowl off (but be careful when doing this because the bowl is extremely hot), and then blow through the bottom of the bowl to cool the bowl down. Then put it back on the hookah and start smoking again taking care to try and put the charcoal in a different area of the bowl that the hookah coals were originally. Depending on your smoking methods or shisha tobacco, you can sometimes get another hour of smoking.
For information on careful heat management and ideal hookah coal usage, see our blog on Hookah Charcoal.