I found some bright red blood in my cat's pooh, she's been constipated, This pooh was a bit runny,?

My little female cat is up and running around and seems healthy, Our male cat had been going quite well with his movements, He was the one I saw get out of the tray just before I found the blood,but I didn't think it would come from him. would he go in his tray with such a mess already there. we always clean it out, directly after they poo especially.there was a trail of it and bit runny and little bit of blood in it.haven't seen any more blood. Our little female has been occupying the tray a lot more since a puppy came into the house a couple of weeks ago, there had been hardly any pooh coming from her since. She is a very quiet, gentle little girl, fairly nervy when puppy is around, but our boy is wonderful with the puppy, he plays with the puppy.