Feeling faint woke me up

Last week I woke up at 2:30 am from a sound sleep because I was feeling nauseous and faint.  I proceeded to the bathroom because I thought I would vomit.  However after being in there a minute, I fainted suddently falling to the ground.  When I came to, my feet tingled and my chest and arms felt very heavy.  After 2 hours the heaviness in my chest and arms didn't go away, so I went to the hospital.  They treated it as a heart attack.  Unfortunately the nitro pill dropped my blood pressure, and again I fainted.  They ran a blood test for a possible blood clot.  It came up positive, but after a chest x-ray and CT scan it was determined to be false.  I spent 10 hours in the ER, running various tests, including a stress test and everything checked out okay.  No blockage in my heart.  My blood pressure tends to run a little low, so I was told to drink more water to keep my blood pressure up.  I am a 40 year old woman, who is normally very healthy.  I still don't understand how this fainting came on, while sleeping.