Cow's milk is not recommended for adult cats, as many of them are lactose-intolerant, and have problems digesting it. 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, if a small amount of skim milk seems to be okay for your kitty, I wouldn't worry about it. I've read some people give a small amount of yogurt as a treat.
Either way, it should probably be a rare treat and not a daily kind of thing.