Gazzilion year old Tewi hasn't had a that time of the month since forever.

Alternatively: She's a rabbit, and therefore has been experiencing an uninterrupted time of month for centuries. All of the youkai rabbits are her progeny.

A few strips later suggest Tewi is lying as usual.

DoctorVisigoth said:

Alternatively: She's a rabbit, and therefore has been experiencing an uninterrupted time of month for centuries. All of the youkai rabbits are her progeny.

I like this train of thought.