Yes, you are experiencing orthostatic hypotension. You need to get your blood pressure checked. If the top number is below 100, you should visit a doctor. Get yoursel a monitor at your local pharmacy.
If your poop is very dark black, kind if shiny like hot tar, not brown, go to the doctor today. If it is very dark black, you probably noticed that you have been constipated for a day or three.
In the meantime, before you stand, flex your feet, calves, thighs, hamstrings, butt, and belly muscles prior to standing. Stand still for a few seconds before walking in case you pass out.
It is only loss of consciousness when you do not know what is happening. Your symptoms during the hypotensive episodes are from hypoxia, or lack of oxygen to the brain. If you are blonde and it happens around your friends, blame it on your hair color and have fun with it.
I take RX's to increase my BP. Otherwise I fall down and go BOOM! It takes me 40 to 50 seconds to regain consciousness according to the people around me. I am bald, so I claim I used to be blonde. Gets laughs every time.