Aussie, it sounds as if you are experiencing Sinus Baratrauma. As I recall, there is a cavity in the head which has a drain hole. If you were an animal this hole would be at the front lowest point in relation to walking head down on all fours. It is in the same position in human or front higher position. This means mucus and the like can pool rather than drain. If the holes become blocked then you will experience pain.
What might help is to bend and get your head down as far as possible. Other than that, read up, see if it fits your pain, then discuss with your doctor.
In my case the holes are free draining. When I have been swimming, bending over water pours out!