How/why does POTS seem to be worse some days then others?

What is the driving force behind pots anyway? Nervous system? And why is pots named after the heart symptoms when it usually has so many sometimes more debilitating aspects and symptoms to it as a whole?

I'm so frustrated with being stuck in one position. As soon as I stand up I get a giant head rush and immediately feel faint. My bp is all over the place today.

I feel like if I could figure out what the main cause is during the really bad days I could find something that would make it better. As long as I sit here I'm fine. But standing up and walking is too much to ask for! Ah!! Prison I tell you, prison!!!!