Bill, since you're a diabetic, I like to think you checked your sugar when this happened. Hypoglycemia in a diabetic can cause a whole slew of neurologic symptoms, up to and including a full blown stroke, complete with focal motor deficits.
More likely, however, is that your extra-large yawn caused the TMJ to slip a little and tickle the vagus nerve. The vagus nerve (cranial nerve X) enervates many organs, and is made up of about 80% sensory nerves. Among other things, it's the nerve that controls the diaphragm (which, along with the intercostals means it controls breathing - and since it leaves the brain above the spine, it's what allows some c-spine injured people to breathe, a little). It's also the nerve that controls the stomache. When you strike the solar plexus, stimulating this nerve causes the diaphragm to spasm (knocking the wind out) and also causes the stomache to spasm (knocking the lunch out).
Another major function is that of heart rate and blood pressure. Your yawn, if it stimulates the vagus nerve, causes your heart rate and blood pressure to drop. If they drop enough, so will you. This is the root cause of vasovagal syncope. Since you do faint sometimes during blood draws, you would appear to be prone to the vasovagal response.
Certainly, since you're now Officially Old, it's worth getting checked, including an EKG and possibly a stress test. But I suspect ultimately this was a benign event.
Actually, what you're describing is transient orthostatic hypotension, an entirely different phenomenon. That's essentially a delay between the need for higher cardiac output and the delivery of same.
