Milk can be a laxative for cats past the weaning age so stopping that may help with diarrhea.
As this is a kitten she likely needs treatment for intestinal parasites as most baby animals have them including worms and protozoan infections such as coccidia and giardia.
If you plan on keeping her I'd suggest making a vet visit with her to have her checked for internal and external parasites, get her started on shots etc.
Most likely being careful with her diet and clearing her of intestinal parasites will work to resolve the diarrhea.
If the vet puts her on medicine for parasites then you will want to totally clean and wash out the cat box the day after the meds are given to limit chances of her reinfecting herself with worm eggs or protozoan oocytes.
Hope this helps you!
Cat Health, Behavior, Care Expert
30+ years cat owner, rescue, breeding, study of behavior & health care