Not is a short time. Do you know what is that you are allergic to? And are you taking any medication? Having allergies make your immune system work more than usual, which makes you tired, which could make you prone to other diseases. So it is important to take medication against your allergy. Mucus don't go into your lungs actually. But the canal might be producing too much mucus itself. Either way it is not bad. If you don't have fever more than 102F than don't worry. See a doctor anyway when you have time.
I can't say for sure but it could just be post nasal drip. do you really think you can actually feel like going into your lungs? If you're really sure about that that could be a problem and maybe cause pneumonia if it went on and on but that's very doubtful so don't get too scared