Hi Curious and welcome to the forum.
It's actually PLT and I assume it stands for Pre-Lunar Tension. She's a werewolf and her 'time of the month' is just before the full moon as opposed to human women who suffer from PMT.
I hope that helps to explain it.
Wyrd bið ful āræd.
"Coffee. The finest organic suspension ever devised. I beat the Borg with it."
- Capt. Kathryn Janeway