Purse Lip Breathing (PLB)

I'm 78, Pr Ca Rxed with IMRT last year. Apparently OK for now. Also have paroxysmal AFib. Also recently dxed with OSA. On VPAP which I kind of hate.
Otherwise, pretty healthy, I walk some (less than I should), play tennis (less than I'd desire).
Lately, out of the blue, I began pursing my lips and blowing out air. Not related to exercise or anything I can think of. It happens 2-3 times a week or so.
When playing tennis, I've had 2-3 episodes in which I feel very short of breath and have to either slow down, take a break or give up playing.
Pulmonologist: all OK, including pulm. function study. Cardiologist: nuclear stress test negative. He says I don't have cardiac insufficiency. Neither him nor the pulmonologist has an explanation for these symptoms.
At 78, should I be content with the fact that I still AM ABLE to exercise and play sports, etc. or should I keep searching for a reason for this?
Anybody out there that can share a comment?
Thanx!

I thought I was in a "problem breathing"or something like that..
I usually post in Prostate Cancer, but these symptoms I'm having shouldn't belong there. Should they?
How do I find a thread on Respiratory or breathing problems?
Thanx

Thank you Traveler and sorry for the mistake...