@judyb200 It sounds like an 'orthostatic' vascular condition. There is such a thing as POTS, a type of arrhythmia that happens when people stand, but it's a tachyarrhythmia which I do not suppose is the case for you. Instead, you have a drop in blood pressure, maybe due to your heart slowing too much on standing, or just prior to.

This is just an uneducated guess from someone with no medical training.