Ya, its pretty simply. You just didnt have enough blood in your system to run everything. When you donate, you lose about an 8th of your total blood supply and it really effects how your body can and does function. I have a similar story, however it wasnt weed that made my loose my vision it was excercise. My body simply didnt have enough blood to cope with the workout so i got sick and passed out. Dont worry, all will be well again in a couple of days.