Why does one nostril get clogged when i lay on my side

That’s because when you lie down your blood pressure changes and blood flow to the upper part of your body can increase,including the blood flow to your head and nasal passageways.This increased blood flow can inflame the vessels inside your nose and nasal passages which can cause or worsen congestion.Also you may just have a common cold which symptoms include nasal congestion,runny nose,and sneeze.You also may have an allergy.Consult with your doctor if it’s constantly happening and if it’s bothering you.Have a great day.