Maintaining supply on period

I have a feeling my period is about to start again. I've had one since she was born (11 week pp now) and last time my supply went down some but I was still nursing. Now I'm mostly pumping (just nursing at night) and I've been cramping and my supply is suddenly down by about half! How can I keep it up while I'm on my period?! I'm not even getting enough per pumping session for a bottle :(