Why won't my kitten stop meowing?

She's about 3 weeks old. She was given to me by a lady that adopted her and her littermates after their mom died. I feed her canned cat food and she eats it without a problem. I usually have her inside of a kennel (for about a week longer, just so she gets used to the house and doesn't run away since she is going to be an outside cat). I've got her inside right now and she's been walking around but will not stop meowing, and sometimes when i left her out to play she also keeps meowing. I took her to the vet and she's completely healthy. What could it be?