Help, black in poop?

Hi my daughter is 2 months and for the past week her poop has had black in it. I'm so worried. She has relux (not on meds) and was taking enfamil ar but we just switched to enfamil gentelease Her pediatrician tested it and said it was positive for blood, but wanted to make sure the formula she was on wasn't just making iron come out in it so she swapped our formulas.. She had been on AR for over a month before she ever had black poop so I really don't think it was the formula. Now it's getting worse. She doesn't seem distraught or in pain. I'm just so worried. Has this happened to anyone else? What could it be? 🥺