DTA,
Hello and welcome back. I am glad to hear that all in all, you have been doing well and found some answers for relief.
I am sorry this concern has your anxiety radar up at this time...I know exactly how that feels.
However, even before reading all of your post...and about
you having the flu recently...
...that was the first question that popped into my head...'Have you been sick with a chest virus?'
So yes, I think what you are experiencing is most likely residual phlegm/inflammation from the flu.
You could try using a humidifier at night while you sleep, or doing some steam baths to loosen up and help expel the rest of the gunk rattling around in there.
...and in the unlikely event it continues, or you get shortness of breath or a fever, see your doctor of course...but I bet it will clear up soon.
Health and happiness,
S.C.