Went blind and paralyzed after smoking. but not every time.

So I used to smoke small amounts of weed in high school with my friends and sometimes my boyfriend maybe once or twice a week. Sometimes I would feel high and everything was fine. But wierd things have happened a couple of times
The first instance I smoked with my boyfriend sitting in his bed and when we got up and went to the kitchen I started seeing black spots and within a few seconds I was unconscious. I woke up in his arms and he said my eyes were open the whole time but I was unresponsive, pale, and had no memory.
The second instance I was with my stepbrother in a hot boxed car and I could feel my muscles twitching and I was thinking he would say something but I was imagining them. I even swore he flipped a quarter and I heard a gun shot. Which he didn't hear.
The third instance was with my group of friends and I was taking my first dab. Instantly I told them I couldn't see and I fell into my friends arm. This was scary because I was still mentally cohesive. I was completely blind and unable to move but I could hear everyone freaking out around me for about ten minutes until my sight came back. This happened again less severely an hour later that day.
I really enjoyed smoking and I am confused as to what happens to me sometimes. There were times I would sit at my friends house and pass the bong around maybe 10 times and I would be completely fine. I'm not sure if it is an anxiety thing or if it is a blood pressure thing. I've noticed that when I get up too fast I have a tendency to black out more severely than other to where I have to sit down and wait 20 seconds for my eyesight to come back to me. But I am used to that happening. I just want to know what it is and how I can prevent it from happening so I can smoke without being scared of blacking out.