This can be SO challenging, I'm really sorry you are going through this. I find that with cycle-related drops in supply/letdown issues, switch feeding can be really helpful. Essentially, you'll switch breasts at the first sign of frustration from the baby, and do this until a letdown occurs. Switch nursing can help speed up the process.
I would also suggest starting a calcium/magnesium supplement for the future. You can read about this and more in this article - https://thebreastfeedingmama.com/breastfeeding-and-your-period/
If she hasn't had issues until now, I wouldn't suspect it's a functional issue on her part.