This has deep vein thrombosis / pulmonary embolism written all over it.
Basically blood doesn't circulate in the lower extremities because of the distance from the heart - when it stagnates for too long it starts to clot.
Then a chunk of the clot breaks off, follows larger and larger veins until it gets to the heart, gets sent smaller vessels in the lungs until it can't fit and becomes lodged.
Really should sleep horizontally.
