Easy things first. The nose has structures called turbinates in them that engorge with blood when you lay flat. They can cause congestion at night. One should first try flushing the nose with saline (there are a variety of otc solutions or even gentle sprays). This can help, if it doesnt then one can try the breathe right nasal strips. They can be quite effective. If it continues, then an ENT md will be helpful.
Dust allergy? This question is important and could be the result of a variety of entities. Your may have hypertrophic turbinates (enlargement of the structures in the side of the inside of the nose). You may have dust allergy, or a nasal polyp or polyps. The remedy as I have stressed frequently is a proper diagnosis. Emphasis here is what is causing the problem. Treatment depends on the correct diagnosis.