First, it may be ideal to check for any possible wounds. Blood can easily be washed out of a cat's fur with a warm damp cloth. If you find any wounds or injuries, take it to the vet immediately who can can prescribe treatment.
one reason could be because it has a wound or skin problem, or it could have ticks or fleas. Many cats do this if they have bad ticks. If it's not any of them and you are worried about it then you...
Cats use their tongues to clean themselves which could be what your kitty is doing; getting rid of dirty fur. Also, it could be because they have a skin infection, bite, or sore. I have dogs, but...