I suffer with this alot, think the fact i fly a lot contributes but i'm always on the go onboard and glug sooo much water, i always lay back to front on my bed when i get to my room with my feet higher than my body to get the blood flowing again properly.
It happens a lot to people who are on their feet all day despite water consumption so maybe that could be it.
But elevate the legs higher than the heart.x