sounds a bit like you are experiencing orthostatics. all this means is that when you sit/stand up, your body is not capable of keeping your brain supplied with the same amount of blood (glassy eyed, light headed, possible confusion)this is accompanied by an increased heart rate or a drop in blood pressure of 20 each (can be one or the other, or both at the same time)
solution: drink plenty of water and talk to your doctor about possible arrhythmias or other cardiac output problems, especially if this happens while you do not change positions...
T.B EMT-B