I think it mostly has to do with not eating than with pot, at least in my experiences. I have a friend that passed out a couple of times, literally dropped on me passed out once, after we had spent the night smoking, but he hadn't eat much all day, all the times. As soon as he woke up I always gave him something with sugar to eat so he could get a little energy.
Last week I was at the veterinary with my cat (I was pretty baked), out of nowhere my vision started to blur, I couldn't stand up anymore, started to sweat like crazy, I thought I was gonna pass out so I asked for a glass of water and after calming down it went away. I had only ate a bowl of cereal that day so i'm pretty sure it has to do with malnutrition.