17 year old female, on heart monitor due to fainting spells.

[Caitlin17]

I am a 17 year old female. I weight about 120 ish and am 5'4. For a few years now I have been having weird accurences happening. No matter what I'm doing. It has happened standing, sitting, laying down ect. To start off when this happens Is my heart pounds, I get a weird feeling go over my body (Almost as like a slight tingly/vibration) my chest, throat/neck get a tight squeezing feeling then I start to feel faint. My eyes get blurry and if I don't lay down or  sit my eyes start to get black and I faint. I have had 4 ECG's and all of them came out normal (doctor said because they havent done one on an episode) I also had an echo done, they said my heart was normal and I was put on a holter monitor over a year ago (but the lady who hooked me up did it wrong) and they only got information that my heart got faster some times for no reason.. So I go placed back on a heart monitor on the 20th of october and since then it has gone off 7 times (automatically) I was just wondering if this could possibly be heart disease and what type it could be...(I've been trying to find some types but I'm obviously not a doctor :p ).
Also after one episode I went to the hopsital and they said I was border line hypothyroid, but a blood test a week later it was fine.. My grandma had suffered a stroke a few years ago and that is the only heart problems in my family.
Thank you so much for your help!