I am not an expert, but I know that green poo can also happen when you have an oversupply of milk (I think bcos bub gets too much fore milk which is higher in lactose). You say you are weaning, so reading between the lines it sounds like you are still bfing but less frequently. If I had to guess I would say that bcos bub isn't bfing so often, she is getting a lot of foremilk when she feeds and not so much hind milk. If this is the case you could probably fix it by expressing a bit before feeding her. It will probably resolve itself anyway as part of the weaning process.