during sex, when man (or woman, whichever you prefere) goes down on the woman when she's on her period. he (or she, whichever you prefere) will swallow the woman's blood clots. hence, clotting.
jean: are you sure, charles? i'm on my period. it's pretty messy.
charles: i don't care. i'll do anything for you.
(charles proceeds to go down on jean, when all of a sudden, kim walks into the room)
kim: charles, can i borrow your- oh my god!
joey: what is it? oh shit!
kim: charles is clotting jean!