This is likely 'positional hypotension.' There is also a syndrome called 'POTS', but that involves a rapid heartrate shortly after one stands up. In your case, your blood pressure is too low (I'm only guessing, and I'm not a qualified medical person). Why is it too low? Electrolyte imbalance, low heart rate, valve problems with poor ejection fraction, an arrhythmia.....several possibilities........ALL of which they, meaning expert physicians, should check out for you.

If you are put on any medication, it's for your own apparent good. If your have heart failure, low ejection fraction, or an arrhythmia, or some other condition that needs correction, perhaps drugs are the answer, and maybe only temporarily. But none of us can possible do more than to guess. You need to present at your doctor's office.