Occurs when a woman doesn't remove a tampon before putting a new one in, the first tampon goes far enough into the vagina that the string can't be seen. The "rotten cotton" is the actual tampon that was left in for too long. It usually has a foul smell & discharge associated with it.
Likely to be used by healthcare providers who see this patient for vague hoo-ha related complaints and on pelvic exam find that they have a forgotten tampon in place.
Josie left her tampon in for five days and now it's rotten cotton.
Says the nurse to her assistant, "Once I remove the rotten cotton, make a run for it!" (to escape the evil smell)