Boomba - I definately remember a few times when my DS had blood in poo. As I remember it was as Tanstar and MantaRay described, from straining and a little fissure. I do remember freaking out a little bit. I'm sure seeing DR on Mon would be fine but just keep an eye out for any other changes in poo or temp etc. I think it would only be from breastfeeding if you had badly cracked nipples.
BTW - he is GORGEOUS!!!!!!