You might be getting too hot while you sleep, that happens to me. Does it go away after you've been out of bed for a while? If so, I don't see any reason for worry. Lying down can cause your sinuses become congested. :)
Do you have a ceiling fan over your bed? If so, there's your answer. Fans blowing overhead even on low speed dry out your throat and your nose. Purchase a product called "Ocean". You can find it at your local drug store, Walmart, etc. It is a little white bottle with an orange cap. Ocean is buffered salt water which keeps the mucous and nasal passages moist and will relieve your stuffy nose.