What could be the possible causes of near fainting?

So I should probably see a doctor.....but here goes.....

When you are fine, then start to feel hot and ill, 50 minutes later spin and lose vision, and have a low heart rate and can notice heartbeats in the chest? Then it goes away after 5-6 minutes and am fine.

I suffer from anxiety so my heart rate is often raised well above what it should be, I get palpitations all the time which I know can lead to extra beats/missed beats, This happens to me out of the blue several times a month, seemingly for no reason, even at rest. Could this be a cause? 

 

I'm convinced I have a serious heart defect, like a faulty valve. 

I have had two near fainting episodes in the last year, the first time I put it down to being hot in the car, the second time I woke up, immediately had to run to the toilet for diarrhea, this then led to a funny feeling in the stomach, which led to me feeling ill, becoming jelly legged, hot, spinning, losing vision, low heart rate, feeling of heart thud in chest (not tightness or heart attack symptom) 

Several years ago I also woke up one morning, hot, and nearly spinning, but the sensation went away and never returned for several years until now. 

All near faints have occured in the morning, one near 7am, after getting up at around 6.15, and the other around 10.30 am, symptoms must have started about 9.50am. 

I don't put it down to something like lack of food or dehydration. 

I am only 22!

What is going on?!