12th Century Female Hygiene
Not surprisingly, since most of the writers of the 12th Century were male, we hear little about what females of any class did while menstruating. The almost total lack of mention of this tells us that this was not simly a topic that men didn't know a lot about, but goes beyond that to a topic that was taboo, or at least very impolite.
The most likely solution for women was to wear a pad of rags which was tied to a kind of belt arrangement. Modern pad-stuck-to-your-underwear type arrangements wouldn't work because 12th Century underwear was quite different to modern underwear, and it is likely that many women wore no underwear most of the time.