Dr B has answered this before, so I will tell you what he has said.
Your bird is either eating something red, like colored pellets or a red dyed toy, or has something seriously wrong and has blood in it's poop.
If your bird is eating colored pellets, please convert the bird to a healthy pellet diet, like Harrison's or Roudybush, so that your bird is not eating a bunch of sugar and chemicals.
If your bird is not chewing on a red dyed toy or eating red pellets or food, then you need to Find an Avian Vet to examine and treat your bird in person right away.