Hi DD2 has done this about once a year (she is now 4) - if she really hurts herself - today she fell off a small table and winded herself - then she goes all floppy and 'passes' out.
It is really scary when it happens - it looks as if she isn't breathing.
When we spoke to the dosctor about it a couple of years ago they said not to worry - but this has never happened with dd1.
Any one else got a simlilar experience?