If i rise quickly from sitting i feel dizzy, nauseous , weak and like all the blood drains from my face..what is this?

Low BP: Your condition is orthostatic hypotension. When you rise from a seated position or from lying down your body has to compensate and tighten your vessels so your BP dissent drop. In your case its not enough and you are sensing low blood pressure. THe causes for this are dehydration anemia and rare disorders of the sympathetic nervous system. Speak to your doc you need to find out why