Frequent lightheadedness and fainting, Doctors haven’t found a cause

(18F, 100lb, 5’2)

Throughout most of the actual fainting I was 110 lb, my weight declining is a new thing which I also am not too sure of the cause.

For the past few years, I’ve been dealing with frequent lightheadedness and occasional fainting episodes. Sometimes I’ll suddenly feel dizzy, weak, and like I’m about to pass out. I’ve fainted multiple times, I’m usually only unconscious for a few seconds before I wake up again. When I wake up, I often feel sick, nauseous and shaky.

I’ve had my heart checked and I’ve brought this up with doctors multiple times. I also go to my yearly checkups and have discussed it there. So far, nobody has been able to give me a clear answer. Most of the responses I’ve gotten have been pretty vague, and I’ve basically been told that they aren’t sure what’s causing it.