Diarrhea and Blood in his stool

My cat, Ringo, 5 months has diarrhea. I saw blood in his stool last night and he seems to be losing weight lately. We just got him back from the vet and the vet said he's okay (we previously confined him because he was fed milk and developed diarrhea when we first had him a month ago.) when we got him back he wasn't as active as before, though he's appetite was good and at first his stool was normal. But once I noticed he had black stool but ignored it because I thought it would just go away. But then, I noticed him pooping more times a day and just last night, his poop was watery and I noticed he also lost weight. What could have caused his diarrhea this time? We never fed him milk this time or anything that could harm him. We recently changed his food (we used to feed him whiskas junior before)to Royal Canin Persian when he got back. Could it be the cause of his diarrhea? Is diarrhea something to worry about with cats? Thank you all in advance for your replies.

PS we are taking him to the vet after writing this post. We really love Ringo and I hope that all is well with him.

I know with one of my cats changing his food too rapidly can cause some diarrhea. Switching to a new food has to be done very gradually. I have had to do it very slowly over the course of several months and he still can't seem to tolerate the new food I am giving him so I am having to stick with 1/2 of the old and 1/2 of the new.

I don't think that would be the cause of the blood in the stool of course. You would have to consult a vet to find out the reason for that. Glad you are going back to the vet. He should be able to find out what is causing the problem. Hope he gets better.

Keep us posted on how he is doing.

Did the vet worm him? Diarrhea  can be caused by worms.  Some vets will just  test a stool sample for worms if they have recently seen the cat or kitten.

Call your vet about the black stool ASAP and check with them.

Is his new food "all stages" or adults? My quick google said Royal Canin Persian is for cats over 12months old.  Kittens are growing and  are using allot of energy so they need to be feed a higher calorie diet.