Why do I get congested as soon as I just lay down. I'm not congested at all before I lay down but then boom.?

For months and months now, I'll be getting ready to go to bed, I'll be perfectly fine until I lay down and all of a sudden one nostril is congested really really badly, but the other one isn't. Nevertheless, it's very difficult to breathe and it's really starting to irritate me. It doesn't matter what I take or what I do, it still gets congested. I've tried everything. And what makes me even more mad is I'll roll over and then sometimes the congestion swaps nostrils. Does anyone know why this keeps happening or what I can to to make it stop? I really need help. Thank you! First person with effective advice gets best answer!