Light headed/'blacking-out' of vision after certain movements

Hey there, I'm a 17 year old male and after certain movements (most specifically back/neck stretches held tightly for a few seconds) my vision temporarily 'blacks-out' and I feel like I'm almost about to faint. I also feel quite light-headed pretty much all the time (although it is the worst in the morning). Taking into account that this problem is probably something to do with blood flow to the brain, I tried standing on my head for a short while to see if the symptoms improved. Afterwards when I became upright once again my vison blacked out and i felt light-headed (worse than after I stretch) - I also had a temporary (for a few seconds) pressure and ringing in my ears. After this my symtoms (symptoms) were the same as always.

Other relevant info:

-about once a year I get quite acute exertional headaches after exercise or anything which increases my heart beat rate. These headaches usually cease after a few weeks  and only reappear on an annual basis (or longer). I have had a MRI scan and nothing of significance was present to indicate any disorder.
-I exercise (weights; using proper technique) about once a week for an hour at a time.
-I have had my blood pressure checked and it is within normal limits (so is my heart beat rate).

Any help would be greatly appreciated!

Gabe