However, If she's not throwing up or having diarrhea, she's probably okay with it. Some experts advise that cream is better than regular milk if you must give it to cats, probably because cream has less lactose than whole or skim milk. But again, a small amount of skim milk seems to be okay for your kitty, so I wouldn't worry about it.
Also, recently, several new lactose-free milk substitutes for cats have been developed, such as CatSip, which may also be found in some food stores and most large pet supply stores. If your cat enjoys milk, she may like these products too.
Hope this will ease your mind.