So we went, we waited, they asked lots of questions, D was surprisingly happy and calm the whole time, he had another poopy diaper with some blood in it (the poop also looked kinda green, mean anything?) while we were there, they took a sample and are running some tests, told us they'd get back to us with the results in about 3 days, then sent us home with instructions to come back if he develops weird symptoms or has another bloody poop. Which seems kinda odd since his last 2 poops have had blood, and I don't see any reason why the next one won't have some as well.
They also told us to have our pediatrician check him out sometime next week, so I'll be calling in the morning to try to set up that appointment.
My guess is he's either developed some sort of food allergy (dairy?) that he's getting through my milk, or his digestive system has just gotten all out of whack thanks to the antibiotics. I think I have some more googling to do, but I think I'm gonna cut dairy out and see how that goes. Unfortunately from what I've read that can sometimes take a good couple weeks before it makes a difference, but we'll see. I think the doctor there was leaning towards option #2, but didn't seem to have many answers.
At least they sent us back home again. I did not want to spend another night at that place.