You don't have to be elderly or bedridden! Just human. Press your thumb against the pad of your index finger and hold for a moment. Notice how it blanches as the blood pressure at the capillary level can no longer overcome the physical pressure of your thumb. That's the exact phenomena going on as when you are lying supine...which eventually produced the ache you describe. The "redness" you describe is the hyperemia (compensatory higher than normal blood volume rushing into the region in response to the period of blood/oxygen deprivation). It is what you are going to see in all instances where, as here, the body part (sacral skin and subcutaneous tissue in your case) is off-loaded. You see this (and it happens) because you are now standing or rolled over (no pressure) and can visualize the locale of your discomfort.

I assure you...it is related to your position and pressure on the area.