Green algae poo

My baby boy is 4 months old, and all of a sudden has the dark green algae looking Poo I've read about as being too much fore milk and not enough hind milk. The poo just started yesterday, but his weight gain has slowed down the past 2-3 weeks. He used to gain around 10-12 ounces a week, but lately it's just been 5-6 ounces a week. I can't imagine what would have changed supply wise, but can this be caused by something I'm eating? Or is it strictly supply based? And isn't it a little late in the game to suddenly develope such an oversupply? Or could it be my supply dwindling? Sorry for the 20 questions, but he's been rather grumpy lately and I'd like to get to the bottom of this.