I can attest to cabbage leaves effecting supply. I had terrible mastitis a little while ago and I'd read to use cabbage leaves. I left them in 24/7 (replacing them often cause they felt great!) my supply dropped so much DS lost weight and we had to comp him. When I told the gp I'd been using leaves he told me they reduce supply and should be used sparingly!! I later read to use them for no more than 20 minutes at a time! Wish I'd known!!
Good luck PZ it's very painful!