Why does one nostril stay congested?

When I lay down at night I always have one nostril that automatically gets congested. I can get up and walk down the hall and it opens up. As soon as I sit down or lay down one nostril will get congested again. I have taken allergy pills but this does not help the congestion. Sometimes my nose will even run while congested. Why is this and what can I do. It is horrible trying to sleep cause I have to breathe out of my mouth and it is very uncomfortable.