What you are describing is what we refer to as orthostatic hypotension, or orthostatic pre-syncope
. Basically this is where standing up causes blood that was supposed to be going to your brain fall to other parts of the body because of gravitational pull. The temporary lack of blood flow to the brain causes dizziness, sometimes pounding in the head, and often palpitations (feelings like your heart is racing). In severe circumstances, it causes syncope (passing out). The body has a natural mechanism involving the nervous system that pushes blood up to the brain when we stand up. When people such as yourself feel orthostatic symptoms, it can mean several things. First, you may be dehydrated, or have low blood volume (if you were bleeding
for example). Having too little fluid in your body means that when you stand up, there isn't enough to go to your head when some of it falls to your legs. Another cause is malfunction of the natural nervous system mechanism which prevents this. Finally, symptoms of orthostasis can be due to a damaged heart valve, though this would be very unusual in someone your age.
Since it is unclear where the problem exists, I suggest that you schedule an appointment with your primary care physician
. He or she can take your vital signs sitting and standing to see if there is a difference. If there is, then you may require a further workup.