What Causes Dizziness, Syncope, Hands And Arms Tremors With Body Unresponsiveness?

my 16year old has been getting dizzy at random times, and she says she temporarly blacks out and starts to shake..especially her hands and arms for a 2 to 15 seconds and at that moment she says she cannot control anything her body is unreponsive, but she can feel the shaking and then she comes out of it.....she remembers it all and it scares her that she will die.. she is on antidepressants Zoloft 50mg per day...but this started way before she started the meds.....otherwise she is a very mellow, stable young lady....can u help to identify this problem?