HI SB,
I have a four month old that had the same issues, also my 2nd daughter had this as well. I find it is when she gets cold her nose blocks up. HAve you tried heating the room at night and seeing if that makes a difference. We worked out that she is warmer through the day which is why it goes away. Still anytime it gets chilly over night and I have not heated the room her nose seems to block!
Have a great day!