The 28-year-old was 2.5lbs over the 118lbs limit on Friday afternoon, but the Liverpool bout will still go ahead on Saturday.
Courtenay explained a predicament on social media that proved entirely out of her control, leaving the Briton in the agonising situation of potentially losing the WBA belt.
“We was on weight and ready to go yesterday,” said Courtenay on Instagram.
“Then last night unexpectedly my menstrual cycle started, which makes women gain weight.
“We had no weight issues during camp and all week I’ve been in a really good place and ready.
“To say I’m devastated is an understatement because this has never happened before, I’m always professional, but this was physically out of my control.
“It’s happened and I can’t change it but what I can do is go out there, get the win and get my belt back straight after that, and that’s exactly what I’m going to do.”