While I do not have PCOS, I have always found that my weight loss efforts have stalled around my period.
For me, i am guaranteed a little stall around ovulation and my period. It feels frustrating because as you said, you are doing everything the same. For me, I know I just have to ride out the stall for a few days and then it's back on track. I'm not sure if that's the same for PCOS since I know there is some irregularities in the cycle, but I would like to think it's not too far off base.
Hang in there and keep holding steady. All your hard work with catch up to you soon!