How to prevent myself from fainting AGAIN?

you probably locked your other knee and when you moved it caused a lot of blood to start rushing to your head. and looking up didn't help you most likely pinched a blood vessel and it cut off enough circulation to cause you to faint just watch how you hold your head and don't tilt it too far back

well the simple answer is to just every few minutes just move your shin muscles and contract muscles in your body whilst remaining still. i think what your getting is pooling in your extremities which is causing a decreased volume of blood returning to your heart, and this could be causing less blood going to the brain and your fainting. remember veins return blood to the heart, and they only can work with muscle contractions, without which blood pools, so ideally just tense your muscles every few minutes to keep blood flow back to the heart normal. either that or eat some salty foods, it will increase your BP for a while. also drink lots of water, keep yourself hydrated. Dont OD on this remedy though, you will have problems...

People who exhibit low blood pressure are usually dehydrated and can faint. The fastest way to recover from low blood pressure is to drink liquid, rather water, juice, cola.

Be sure to not lock your knees. And eat and drink enough before.

Try drinking something really sugary and fizzy before you start, and if you still start to feel dizzy just try to relax and take deep breaths.