If you'll sit here right now and repeat what you
saw her do, and think about it
a minute, it's like she was getting ready to sneeze
. When a person sneezes, they act like she did and sort of hold their breath. So, maybe the wind was knocked out of her and she was trying to get her breath. That would be the least unusual thing I could guess.
If you saw her hit her head or if she still has a sore place on her head when you touch it, then you might want to take her to the doctor.