Couple of thoughts -- if Marley is straining when she poops, then blood could be from that. Also, if she drips any urine when she poops (a common co-occurrence), then blood could actually be in urine, not stool. Definitely something you should make notes about (frequency, amount, etc.) so you will have all that info when you go in for Marley's check-up. My vet always chuckles at my lists but he has gotten used to them and it prevents me getting home and realizing I forgot to mention something or ask a question. If blood increases in frequency or amount, you shouldn't wait until visit.