My roommate's cat had a litter of kittens 3 weeks ago.?

Update: A few days ago a neighbor brought us 3 kittens that they found in a broken down car that is in their yard. Our cat adopted the new kitten, even though they are about 1 to 1 1/2 weeks younger. Now she is trying to hide the new kittens. Why would she be doing that?
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