What does foamy EBF poop mean?

[Mom3B1G]

Hi! Yesterday, while I was changing DS (almost 4 months) diaper, he was not quite done, and he pooped out foam. I am not kidding, it looked like the foam soap, though definitely not white LOL! He is my 4th baby, and I have never seen this before (may have happened, but I didn't see it, I suppose). He nurses very well and has gained 3 lbs. every month so far. Yes, he's huge! He is the size of a 6 month old baby already, so we would not expect to have health issues or allergies....? It's weird. What causes it?