Ive had this happen.

Particularly when kneeling and working on something for a period of time. When you stand back up there is a rush of blood to the legs below the knees and this creates a temporary pressure drop to the brain like bird says.

Seems to coincide with smoking mj with me. Lasts about 20 seconds but can sometimes make vision look like static on a tv set and I have to grab something momentary. Not always, but know what your talking about.
Fairly common with mj smokers, called "head rush".

I smoked with a guy one time and he was kneeling down working on his truck, stood straight up. Fell backwards stiff as a board and bonked his head good on the concrete. After watching that always make sure to grab on to something.