It’s not one condition, basically when your respiritory system is irritated it produces mucus, that’s why you get a runny nose when you have a cold for example, but anything going weird there can cause this reaction, infections, tumours, anything. When there’s an overproduction of mucus you might not notice when awake and upright as it drains away but when asleep laying down it can build up in certain areas. Your description of having to clear your nose and tightness on the lungs sounds like this could be whats happening and I feel like someone with a medical degree should have a look just to be sure it’s nothing sinister.