I was about to post that we had the feeding situation worked out and that the bleeding thing had resolved. Then I changed another diaper with blood in it. Damn it.
We have, as before, no idea what is causing this, but we're back in the process of ruling out all the scary stuff before we conclude that it's just irritation due to increased feeding/stool volume.
Meanwhile, here is a picture of a cute baby, who is being charming and happy through all of this.