How do you know it's hay-fever..?

Hay-fever does in fact cause sinus congestion which over time can lead to chronic sinusitis. I suspect this is what is causing your symptoms. Lying flat will make sinus congestion worse. I suggest you see your local pharmacist for a decongestant like Sudafed. Also ask for a steroid containing nasal spray like Rhinocort to treat the problem at the site. Feel better! See your GP if problems persist, or if you develop fever, or the headache is associated with neck stiffness and/or rash.