When standing up, i sometimes have a brief sensation of not being able to bear my own weight, what is it? this only happens occationally.

Orthostasis or: Orthostatic/postural hypotension, blood rush or dizzy spell), is a form of hypotension in which a person's blood pressure (BP) suddenly falls when standing up/stretching. Systolic BP falls at least 20mm hg & diastolic BP of at least 10 mm hg when a person stands. It is quite common, may occur briefly in anyone, & particularly in the elderly, those w/ low bp, & on certain meds.