Same here. I have horrid periods in terms of pain, length, and how heavy they are. I give myself flexibility. If I make my fasting goal, typically OMAD or 23:1, then great! If I make it to 20hrs, awesome. If I do 12hrs during my worst day, fine too. As soon as the worst is over, I get back to my routine. Be kind to yourself, hormones are a b**tch!