Kevin, I experienced this same symptom as a youth.  One pulmonary
specialist I spoke with several years ago referred to this condition
as "sighing syndrome," which is viewed by many, but not all, to be a
variant of true hyperventilation syndrome (NOT chronic ventilation
syndrome, which is a pet theory diagnosis of those who believe that
*all* asthmatics are *always* overbreathing).  

There is no known cause of this condition.
If this is your only breathing symptom, it probably isn't asthma.  A
pulmonologist's office would be a good place to start.

In my case, the problem resolved with a little chiropractic care, but
I had limitated motion in the joints of my upper neck (due to several
head and neck injuries).  I believe mine was a mechanical problem,
which may have irritated my brain stem (where the respiratory center
of the brain is), hence, the annoying symptom.  Mine is a completely
anecdotal experience, however, and may have little or no relevance to
your particular situation.

What did you do differently (physically, environmentally) two or three
weeks prior to the onset of your symptoms?  What has changed with your
health besides these recent intermittent attempts at sighing?

It is a sign of acidosis ..

Who loves ya.
Tom