maybe its cos you ran up the stairs too fast after being in you're basement for too long.
changin altitude quickly, even slightly, can make you blackout n shit.

Cos one time i was at my cousin's yard just chillin and i musta been lying down for a long time and i got up too quickly and blacked out and dropped. only lasted for like 10 mins but i was lucky i was in his bedroom and landed on the bed or i would've hit the ground....