one of my siblings worked black friday in a store as a cashier. she was telling me it was very common for workers in her store to go through this because they could not just take a break any old time they wanted to sit down, get something to drink, eat. they had scheduled breaks and some chose not to take those breaks. this caused them to have this feeling like this. their job called it muscle fatigue, stress, dehydration and bordering on fainting spells.

my suggestion next time you need to work such long hours on your feet and under stress of black friday/ holidays maybe you can pack "pocket snacks and quick high on electrolytes drinks like single serving gatoraide, sunny delight, fruit drinks and waters, that can be kept on your work stations shelf. that way between check outs you can fuel up and do a few quick leg stretch exercises to get the blood flow going again. this is what helps my sibling from having this feelings that you had.