3 week old kitten with bleeding bum

[lovemykitties]

I am fostering a kitten. I have had him since he was 4 days old. He is now 3 and a half weeks old. For the last couple of weeks I notice there is a little bit of bleeding on his anus when I help him eliminate. I took him the the vet for a check up when he was a little over a week old and she said everything was fine and didn't seem concerned about his bum. I was just told to put vaseline on it which I do but it doesn't seem to stop his bum from bleeding.I called the vets office again to tell them that his bum is still bleeding and they said it was common with constapation but I don't think he is constipated he goes several times a day and it is a yellow brown color and not hard but not runny either. He is eating kmr not the powdered kind the ready made.The vet did give me worm medication for him and said I could use it at 2 weeks but said she had never used it on a kitten that young so I am a little apprehensive about using it. Also when I make him go poop it seems like it hurts him. Does anyone know what it could be. He is eating and sleeping and playing and acting normal and gaining weight. Even though the vet doesn't think it's anything I am still concerned.