After orgasm my neck gets noticeably fatter. Like when I retract my chin, there's this belt of skin below my jaw (aka double chin). Also my jawline becomes less visible.
When I don't have POIS symptoms, my jaw looks normal, and it's much harder/impossible to create a double chin.
To an outsider, the difference may not be so apparent (as far as I know, nobody else has been shocked about my sudden fat neck), but to me it is.
It's very annoying.