A common cause for diarrhea is intestinal parasites. Treatment is antiparasite (worm) medicine, and could be purchased OTC in some locations, at pet stores.
Milk can sometimes cause diarrhea in adult cats since adult cats can become lactose intolerant as they grow older. You can try lactaid milk, in which the lactose enzyme (lactase) has been added to the milk so it is easier to digest. Canned cat food without any grains is the healthiest kind of diet for cats.
It could be something far more serious of course in which case she really should be evaluated by a veterinarian.
About a month ago my cat Riley, had really bad diarrhea, I called my vet and he said Riley has worms and do go and purchase deworming medication at the pet store or grocery store and follow the directions and if this did not work to bring him in, the deworming medication work and I also gave him children's Caopectate, and Riley is feeling much better and now I will be deworming him every 3 months.