How old are you? It depends on your age -- if you are 60 or older, there is a possibility that you may have vascular disease that supplies the brain or in the brain itself -- and if you are less than 40 and generally in good health, most likely is volume related or anemia -- but regardless, you need to see your doctor and get checked out -.
Talk to your doc. When you stand up your BP may drop. If it takes more than a few seconds to increase as with certain meds, you may feel light headed. In older people with atherosclerosis looking up can slow the heart beat causing lightheadedness (carotid hypersensitivity). You may simply be having benign positional vertigo in which case you feel an imbalance or room spinning. All of these can be remedied.