I would guess it would be Harry. If his tummy is upset it's a good chance he'll have loose poop too. However, can you maybe isolate both he and Allen to see for sure? It could just be something Harry ate that's making his tummy upset. Have you given him the hairball stuff? Other than this, I'm clueless, but sure hope you find out who the culprit is soon, and find out why there's blood.