Blood in stool suddenly

I got my male baby a week ago when he was 3 weeks, now 4. The mom abandoned the litter. Since the first day he has had very watery diarrhea. I brought him to the vet today and she prescribed antibiotics. He had two doses so far and after feeding him just now his stool was less watery but when I was wiping there was a bit of red blood. He also didn’t poop it out I just noticed it was coming out and wiped it off but he did not use the litter box or anything. His rectum looks also more swollen and red than it was and he cried a lot more than usual, tried to bite and hissed a little when I cleaned him which makes me think it hurts. The vet said that although it did look red that it was from the diarrhea but now to me looks worse. Any thoughts on what could cause the bit of blood? The vet is closed tomorrow so want to ease my anxiousness until I can call on Monday. Thank you!