Need Advice

So my baby is almost a month old now and for the past week we have been noticing blood in his stool. We took him to the doctor who confirmed the presence of blood. We are supposed to get an ultrasound of his belly done at some point to make sure there’s nothing wrong internally. He had a diaper rash and I thought that’s why he was bleeding but the rash is gone and he is still bleeding when he poops. He strains a lot too. Can anyone help explain possibilities as to why this is happening?! It’s very concerning. I need all the advice I can get.