Sometimes when i get out of bed or stand up i feel very dizzy... i even fainted once, what could that be?

See your doctor: Benign condition such as transient orthostatic hypotension refers to a drop in blood pressure that occurs with the position change of either lying/sitting to standing. Standing causes the blood in the body to move downward due to gravity and pool in the legs. This results in decrease blood to the brain and a sense of fainting. Medications can also cause similar symptoms. Please see your doctor.

BP may be low: Your blood pressure is perhaps dropping when you stand up suddenly making you feel faint. A review of your medications is first and foremost essential to see if this is a side effect of one or more medications. Have your blood pressure and heart rate checked in the lying, sitting and standing posture for starters. Taking your time, sitting at side of bed and moving your legs might help.