Blackout from smoking?

ive never had problems with smoking until i got crutches i now have almost passed out 3 times, the three times i have been standing up on my crutches. i was with my friends smoking a blunt standing next to a tree i started to go into a tunnel vision and then it went away. the next thing i kno it came back but much stronger... i went pretty much completly blind until i sat down on his chair then i was 100% ok i smoked another blunt after and it was fine. it was just weird i dont understand why this happens? im completly fine once i sit down for about 30 seconds this has happened 3 times but ive smoked inbetween these times and not passed out i have a feeling the crutches may be cutting off circulation? but idk hopefully it will go away once i get off crutches