I actually get this pretty frequently anymore. Somedays I have difficulty all day long and other days I will feel fine and go out to do something and then within a few hours I am having difficulty.
For me it can happen due to several reasons:
Anxiety - I get anxiety in the evenings, if I am in small spaces (like my bathroom) or if I am in a house that is hot or if I am in clothes that make me too hot.
High Blood Sugar - If I have had a lot of sugary foods and my sugar has been out of control I will get shortness of breath. Also if I have things like Coke in general my breathing is more restricted and I cough more - even if my blood sugar is normal.
Humidity - If it has been raining alot and the weather has been bad I tend to get tighter and have more congestion than usual. This happens more often in the fall and winter for me because the weather changes so much from day to day - 40s and rainy or windy one day and 70s and sunny the next. The jumping back and forth between warm and cold does a number on my breathing.
Also a big one for me is when I do stuff I tend to start out really fast. My lung function has taken a hit this past year and it wasn't as gradual as it has been in the past, so I still mentally am used to functioning at one level and have to remind myself to slow down sometimes. When I force myself to go at a pace that I am able to handle I do not have as many problems. However when I push myself to walk quickly or to carry heavy things and so on, I get short of breath very fast and end up having difficulty.
Sometimes too I will do an extra treatment and CPT and I am able to cough up a TON of stuff if I am really short of breath. So maybe you could try that and it might help too.
Take Care,
Lindsey