Originally Posted by optik_nerve

I'm no doctor but after doing EMT training and being in the hospital setting this is a classic case of Orthostatic Hypotension. When going from a sitting to a standing position the systolic will drop to make up for the lack of blood flow to the upper region of the body. You actually lose 700 mL of blood from the thorax when standing up so it triggers mass vasoconstriction to bring blood back up to the core organs i.e. brain, heart, lungs.

I don't know if the diastolic not changing is normal but a 20 mmHg difference in systolic is a significant difference. How old are you? Are you taking any diuretics, blood thinners, anti-depressants or MAOI's?

If the problem persists just go see your physician.