What is this in my son's poop? *TMI*

He has a doctor's appointment tomorrow so they told me to take pictures if it happened more than once. He has a bleeding diaper rash on his bum but when I changed him this morning his feces was mixed with what looks like mucus-y blood. It's not from his rash. My poor little man has been fussy all day long. He is formula fed so I hope it's not making him sick. Anyone ever seen this?