TMI WARNING: Mucus in formula fed baby!?

Ch

I’m a FTM and I have twin boys. One of my twins earlier tonight smelt REALLY bad like cheese. The boys are almost 4 months old. I check his diaper and this is what it looked like. What could this be from? I’m gonna call dr tomorrow but their poop has NEVER smelt like that. It was so bad his whole body, blanket, onesie, and pjs smelt.