Period.

"Ugh," thought Karyn, "I can't believe she just pissed through us. Sarah is a sadist! I just hope she doesn't do anything worse."
"Well, pussies, now that that's done with, we can start the real fun! I wish that my menstrual cycle would start right now!"
Karyn gasped- or would have, if she had lungs. She suddenly felt nauseous.
"Oh no!" thought Jon. "Now I'm going to start bleeding!" Sure enough, he felt a warm, thick substance start oozing past his lips. It felt like he was drooling, except it was coming from his throat. He saw a trickle of blood run down Sarah's leg.
"And of course, as every young woman knows, it is necessary to use hygiene products to hold back the flow. So, for the next few days, enjoy the feel of..."