I'm not a doctor, so this is not a diagnosis, but after years of lung issues, I would guess that what you are seeing is progressive clearing of your lungs - first the "loose" mucus was expelled, and next, the plugs worked their way out and came out. Now you are seeing the old mucus that was deeply seated in your lungs being expelled.

Since you are new to airway clearance, take it easy on "huff coughing" during and after the nebs, and don't try to move it all out in one go. It has been building up for months (at least) and will take a long time to clear. Otherwise, you may irritate your airways, your throat, and even make your ribs hurt.

If the mucus becomes all green, or you have a fever, don't wait until April, call the doc and describe what you are experiencing.
Sue