Dear XXXXXXXXXXXXX:
You appear to be describing orthostatic hypotension, a temporary (usually about 3 minute duration) decrease in blood pressure that occurs with standing, usually after prolonged rest. The pressure drop can be significant enough to lead to fainting.
There are many possible causes, including: lack of blood volume; certain medications; dehydration; prolonged bed rest; Addison's disease; diabetes; etc. Treatment depends on the underlying cause.
A few suggestions to minimize effects until you are able to see your doctor:
Take a deep breath and flex your abdominal muscles, then slowly sit up and/or slowly stand. Maintain adequate fluid intake. Eat multiple small meals instead of fewer large meals, and take extra care when standing after eating. If you feel you are about to pass out/fall and cannot grab onto anything for support, quickly adopt a "squat position."
Doctor (MD)
In practice since 2000.