I feel a little dizzy and eyes blackout when i get up from sitting or lying down. what can that be?

Many causes: There are a number of causes for orthostasis - when going from sitting to standing and your blood pressure does not compensate for gravity. See your physician to have your blood pressures checked. They will also likely do an ekg and some blood work to make sure you are not anemic or have another correctable condition.

Med eval: This sounds consistent with postural hypotension. If occurs if gravity pools blood in the lower extremities causing less blood flow to the brain. Please have it evaluated for accurate diagnosis soon. In the mean time - hydrate well and go very slowly from lying to sitting to standing. This can be dangerous because you could be injured when falling.

Cause of dizziness?: Orthostasis - that's the general term for your blood pressure dropping after standing from a seated or horizontal position. This causes dizziness and lightheadedness for a few seconds or so. Certain meds can cause this as could an array of issues. Set an appt with your pcp soon to discuss further. If your symptoms worsen, go to the er. In the meantime, drink plenty of fluids and get up slowly.

Blood flow: This is called orthostatic hypotension. It is when you change position from sitting or lying down the blood rushes to the lower part of you body due to gravity, causing low blood flow to the brain and your symptoms. This can be caused by many things, blood pressure medications, dehydration, occluded blood vessel(vetebral basilar artery syndrome), intestinal bleed, etc. You need to see your pcp.