is this a concern?

Hey all, I need some help. My son is 3.5 weeks old and the last two days he had been having red diahrrea, he's been eating fine but I've noticed and increase in fussiness and gas. I thought it was a dairy allergy based on some research. So I started to feed him diary free formula. His poop is sill watery but now brown with red streaks of blood. No fever that I am aware of and is eating fine, just more spit ups than usual but no vomiting. I called the pediatrician two days ago when the diahrrea was red and they said to keep and eye more on the vomiting. But I'm concerned about his poop and increase in fussiness and irritability 😔 anyone have any idea???