Ay, nangyari na rin yan sa amin dati with my youngest! Black stringy stuff in poop can be normal sometimes, it's usually just undigested food or mucus, especially if baby is eating solids na, pero i-monitor mo lang if may ibang signs like fever, diarrhea, or if baby seems uncomfortable. Better to message your pedia just to be sure kasi bawat baby different, at gusto mo peace of mind na. Hope it's nothing serious, pero don't stress too much muna, most of the time it's just one of those things!