Isn't it interesting? Apparently it's quite normal -- I've read some studies calling it "very rare," but we actually get lots of questions on VP about a similar phenomenon, so I suspect it isn't as rare as some experts think.

It appears most common in females taking progestin (i.e. in hormonal birth control), especially when starting -- I can't dig up the link now, but progestin was linked to a certain process in the uterus that makes shedding the uterine lining cast more likely.