The sinus, ear and throat are all connected. Seeing as it occurs when you are lying down I would assume that it is cause my pressure building up. It maybe something to do with your sinus or it might be because of all the extra blood you make when pregnant. All sorts of funny things can happen with that extra blood, with Dominic I used to get blood noses all the time.
Still it is painful and a worry so I would go back to the doctor and get them to do a proper check of your ears, nose, sinus and throat. If they can't find anything you could ask to be referred to an Ear, Nose, Throat specialist.
Hope you are better soon .
Congratulations and best wishes for your birth! Not long to go now
edited for spelling
Edited by Saffron