Do not worry about not recovering from the headache. What happens after FESS or any surgery is that the layer of the nose inside gets torn at places and then it starts to heal some what nicely and some develop scabs or crusts. These crusts along with nasal secretions then create a blockage similar to what you were operated for. This does take time to clear and if your doctor said three weeks then do wait for some more time. I would suggest you douche your nose with warm water mixed with a pinch of salt and three fourth spoon of baking soda - do it daily for a week - ten days. It will clear your nose. The medicines that have been advised point to the fact that the doctor thinks that the headache is a vascular one, akin to something like migraine. Do not worry, take simple pain killers like calpol or nimulid and clean your nose. Keep a positive attitude and you will do well.