Stand in front of a chalk board and draw a straight horizontal line from the left side all the way over to the right side. Now go back to the left side and draw speed bumps and dips for a few inches. The suddenly draw mountains and valleys. The go back to the speed bumps and dips.
This is called Homeostasis and we all have this pattern through out the day. The speed bumps may be getting everyone in the van and off to schools. The dips may be the cup off coffee that we have once we get back home and just stop to take a breath. It's a pattern of equal reactions that take place to stimuli. A stressful time is the bumps and mountains and the relaxing and recuperating is the dips and valleys.
For instance; someone chews me out for a choice I made that I feel I thought out and made a really good choice. I get really involved with defending myself, I get higher blood pressure, my heart beats faster, I start shaking, I am fully emotionally involved in my argument. Then when I leave I cry, I talk out loud and think of all the witty things I could have said, I start calming down, I get distracted, I start feeling sleepy and really want to go home and take a nap. When I finally get there I can't think what my name is or what I even want to eat for dinner. I go to sleep and don't move all night and wake up groggy but more refreshed as the day goes on.
That would be a mountain then a valley with a slow climb up. Homeostasis is this cycle. I have a friend who is bi-polar. She has mostly manic attacks. She has mountain after mountain without ever going all the way down on the valley side so she never is able to recover and gets worse and worse, more out of control.
This little one you are describing is just having a mountain and then having a valley. Her little body can only take so much stress then it is shutting down. She needs to find better ways to communicate what is wrong and be able to recognize she needs to be able to calm herself down. Hopefully someone will teach her to use her words and help her learn the words she needs.