Same exact thing is happening to me, I just started my cycle this weekend and my supply plummeted 😩
I follow a lactation consultant on Instagram and she posted that you can take a daily dose of 500 to 1000 mg of calcium and magnesium supplement from the middle of your cycle through the first three days of your period to help to minimize any drop in supply. I was also reading that this just happen on the week before the period. I also got mine and my baby was just eating more often one week before. For me the worst thing was the soreness on my nipples.
If you have a pump, pump while feeding her. This tricks your body into thinking you're feeding twins. 10 minutes on each side, every 1-3 hours. Give her the pumped milk too. This was the feeding plan we put my baby on when it was discovered he wasn't gaining weight very fast and my milk supply has been part of the problem. Good luck!