From time to time, we all take a deep breath after breathing normally
for a while.  For the past two days, at the very end of the exhalation
from the deep breath, I get a dull, very localized pain, in my right
forearm about 2 in. above my wrist.  A few days ago, I lifted a 20 lb.
container of cat litter and handed it to someone at arms length from
me - I did this twice in a row - and it maxed out that arm to the
point of pain.  My arm does not otherwise hurt, and is fully
functional, and I do not feel any discomfort in my neck, but when I
extend my right arm and flex the muscle, I feel dull muscle pain in
the muscle in the front of my shoulder and in my right shoulder blade
area.  If I take a second deep breath immediately after the one that
caused the dull pain, the pain doesn't happen.  I am a 40 year old
female and I am aware that any advice given here is not to replace
actually being seen by a doctor.  Any thoughts?  Shelley

As long as you can rule out dangerous conditions such as cardio
problems, I would say you are, sorry to say, just getting older and it
will probably go away with rest.

Cheer up, a lot of us are exploring this territory well ahead of you.