do u think maybe ur dehydrated? are u taking any kind of meds? do u usually have back pain?
That is alarming. You should see a doctor about this as soon as you can, as it is possible that this is some sort of seizure. Blessings - Blu
My brother used to do that as well but he was having panic attacks from social anxiety disorder.
As you were not having an anxiety attack, and just collapsed, I would suggest you see your doctor to make sure there is no underlying problem with your legs and spine or any neurological disorder.
Best wishes.