Pressure in forehead and left arm numbness whenever I stand up. What is that?

Headache arm pain. Not enough data for a good answer. How old? Weight? Any sickness (htn, diabetes.). Standing up makes the heart pump stronger and faster so blood is rushed to the brain. If heart is weak and sick, minimum increase in activity can overwhelm it. Give us more data about you. Need to see your doctor soon as possible. Do not wait for healthtap answers when you may have a severe problem over other prob.