The Case: Kristen, running for a frisbee, jumping into the air to stop said frisbee from entering the eager hands of an offender, Kristen on the ground, running around, unable to breathe.

I got the wind knocked out of me really hard today, and my medical mind started wondering what was going on.

The Diagnosis:

Getting the wind knocked out of you is a phrase that mainly refers to a kind of diaphragm spasm that occurs when sudden force is applied to the abdomen which puts pressure on the solar plexus. It results in a temporary paralysis of the diaphragm that makes it difficult to breathe for a short period of time.^[1][2] It can also occur from a strong blow to the back.

When the abdomen is struck, a large difference in pressure occurs across the diaphragm. The diaphragm then stretches, which also stretches the diaphragm’s nerves. The resulting mechanical force puts the diaphragm into a muscle spasm, comparable to having a charley horse in the leg. It takes a few seconds for the diaphragm to relax again before breathing can resume normally. Martial artists are often taught to breathe out heavily when struck in the stomach, to minimize this effect.

Thank you wikipedia.  You learn something new every day!