As far as I know just about every organ loses capacity over time; at the same time the RATE at which it decreases can be of concern if it seems too rapid a loss. U seem to wonder if u lost over 5 months because u did check it then?

Regardless, I'd look into lung diseases category by Mayo to get a sense of the larger picture.

Most important, IMO, is how you can NOW on with a lifestyle that can give you a level of your lung health that helps you with the life you want.
For me I'd like an independent life so I walk up many floors (upto all 13) in my bildng with the last few with My top rate my organs allow today, often with grocery bags; often Without looking at what floor I am cuz every one is a better gauge of themslves than some general guidelines.