Why is my heart rate 100 when standing up?

I do not know: Maybe you are a little dehydrated and need more circulating fluids. Drink more fluids (water) and see if that helps and see Doctor if the HR stays elevated. Best wishes.

....: when you satns up from a seated position the vessels in the body vasodilate causing the blood pressure to slightly decrease which in turn causes a reflex tachycardia ot elevation in pulse. talk to your doctor if you have dizziness, lightheadedness or palpitations... or if you have tachycardia at rest.