Smoking cigarettes causes your arteries to narrow, which can cause high blood pressure, which can cause a bloody nose. so if you smoke cigs, thats why.

now this is the interesting part..

smoking weed while standing lowers your blood pressure, but smoking weed while sitting raises blood pressure. so its possible that mj is the cause.

also, not related but interesting, when you stand up after toking up, since your blood pressure is raised above average, you experience orthostatic hypotension, which is a drastic fall in blood pressure, which causes dizziness and blurred vision.

god i love wikipedia.