i recently tried some simple meditation exercises but in a diffrent posision

instead of crossing my legs
this is hard to explain standing with your knees bent while leaning on your toes

i tried this because i use this position a lot and in the beginning it hurts if you do it too long

but when i stood up and opened my eyes i saw the area i was in (my room) and then
i couldn`t see anything but black for a second and everytime i do this it happens again

so is it normal that you cant see for a second and feel something in your head after meditating(like when your standing upside down)?