This is a most difficult time, when they are babies. It could be something as simple as allegies BUT you don't know that. Here is what I did when my Rachel was like what you described... First of all, we had our own stethescope
I watched and listened to her sleeping, with and without the stethescope. (no not every minute just to assess) Does she sound congested? Where do you hear it? Nose? Upper chest area? Is she breathing through her mouth?
My first bought of "illness" came when she was about 5 months old. Background... CF was 5 mo. new to us, Anna was diagnosed just 2 wks before Rachel was born AND my husband was deployed to Desert Shield/Storm 6 wks after Rachel was born. I discovered that she was all stuffy and congested, but mostly in the nasal area, no fever. We (my mom and I) inclined her mattress and I am sure gave her some dimetapp(back 19 yrs ago when it was still available). When this did the trick, all was well, but if it didn't seem to help or I started to hear or question if I heard any sort of congestion in her chest I would call her peds. clinic for an appt. We didn't see the CF doc very often, every 4-6 months back then. Not to mention I'd locked up our base house in NC and went home to TX and the CF clinic was in NC.
Here's what I would do now. If the CF clinic said to call with any illness, even a cold, I would. Really you want to avoid the ER if you can, ER's are a breeding ground for illness. They can evaluate her symptoms over the phone, probably/maybe tell you to watch her over the weekend or just the next night or two for fevers and/or breathing difficulties, suggest a OTC med to try (depending on her age) if all she's really having is a icky nose, no fever and sounds good and you are already doing increased nebs.
Now if she's having or showing signs of breathing heavier or a croupy sounding cough, then I would call and tell them she needs to be seen before the weekend. No ER, this does not sound like an ER visit, and ER is not a clinic it is for emergencies, which by your description this does not sound like. Not to mention you are going to see an ER doc and not a CF doc who will assess the stituation after you have waited for a while in the ER, THEN they will call the CF doc. Unless they have a different sytem set up at your hospital where they call in the CF doc as soon as a CF patient arrives.
I hope this helps somewhat. If she's sounding cruddy, having difficulty sleeping, call the clinic. That is what they are there for. They know you are new to this. The last thing you want is for things to settle into her chest because then you have a potential infection waiting to happen. I wouldn't let things go longer than a night or two and never over the weekend if I was in doubt.