Thanks for all these additional details.
I am sorry to hear about your daughter's problems.
It sounds that she has suffered from a Syncope. It occur when brain doesn't' t gets enough blood flow and one lose consciousness.
Usually a slow heart rate causes a drop in blood pressure, this reduces the blood flow to the brain.
Possible causes are-
-Heart disease (decreased blood flow to the heart or irregular heart rhythm),
-Low blood sugar,
-Blood pressure imbalance.
At her age, i would not be worried for heart disease.
As you said, that she has just taken meals, so it can't be low blood sugar level.
May be blood pressure imbalance or panic attack is the most common possibility.
It is also possible, that this could be related with fatigue and stress.
A clinical examination should be necessary.
Though chances of getting any serious issue are slim.
But for peace of mind, it is better to get a clinical examination done.
Please consult her primary doctor and get it clinically evaluated.
Hope this helped
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