Brief Answer:
Needs comprehensive neurological workup

Detailed Answer:
Good evening. If the timing of 21 minutes is known to be absolutely accurate then, there is little chance of this being a cardiological issue and I really would not waste a whole of time or resources on getting a cardiologist involved except to get something on the order of a simple EKG, perform a cardiological evaluation and echocardiogram just to demonstrate and be satisfied that she doesn't have a COINCIDENTAL cardiac pathology.....but more importantly she needs to seen by a pediatric neurologist and probably an epileptologist given the length of what could've been a POSTICTAL phase of recovery following a seizure attack of some type.

The fact that she has no recollection of anything beyond the time of passing out means that her brain was in a very low state of alertness and function and this would be consistent with an epileptic type of seizure though it doesn't sound like it was a tonic/clonic type ("grand mal"). If she suddenly just fell to the ground without any warning and no shaking, twitching, or jerking then, she may have had what is called an ATONIC seizure which just means that the body suddenly goes limp and the person loses consciousness. Following seizures people often complain of headaches, extreme lethargy and fatigue, and muscle soreness in the limbs.

You may check and see if the school or anybody at the event was taking any video at the time with your daughter in it since it would be helpful to her doctors to see how she was just before this happened as well as while she was on the ground.

She needs an EEG and quite possibly a prolonged sleep deprived one along with a metabolic and hormonal assay to check thyroids, pancreatic function, and blood counts to make sure she has plenty of oxygenation and that there is no occult infection brewing that could have brought this on.

Imaging wise she will need at least an MRI with gadolinium contrast and fine temporal lobe cuts which would be looking for some evidence of mesial temporal sclerosis (MST). She may also benefit from a functional interictal study such as SPECT or PET scan looking for epileptiform activity that could be correlated to her MRI and EEG. I would also order an Ambulatory EEG for 72 hrs. (AEEG) which will allow her to take the unit home and have her recorded during normal as well as sleep activities.

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