I had this with DS many moons ago (it was a river of green poo, at the Drs, whilst she was checking his hips AND we forgot to bring a spare nappy - new parents eh!)
Anyway, I over heard a BF counsellor (and midwife) telling another mother that it can be too much foremilk, not enough hindmilk - apparently foremilk has more lactose in it and can upset baby's tummy, possible causes maybe switching breasts too soon, so baby gets a double blast of foremilk OR topping up with formula after a short breast feed (that's what did it for our DS for sure)