What Do Sudden Blackouts And Breathlessness Indicate?

Question : today in the morning when my 13 years daughter woke up,and went to the bathroom to get ready for school,we heard a falling noise from the bathroom and rushed to bathroom and came to know that she had fallen,she unlocked the door and came out,explaining that she had a blackout and she feel,she became motionless after that and was feeling breathless and a did not speak for 2 minutes,we settled her on the bed and just gave her some water and some sugar!she was normal after that and has gone to the school!she is nearing her final exams in a couple of days and is tensed about them!!what could be the possible cause of this incident,do we need to Investigate?she generally gets this fear of exams,please suggest remedy for that also !when I was droppi g her to the school she was normal only worried about her exams?also we were lucky she didn't get hurt when she fell although she doesn't remember anything what happend how she fell!