After reading Whisphershadow's post I I agree. Take the kitten to the vet to see if there is anything wrong. You could then let the rescue know just for their information in the future. My first step any time I bring home a rescue is to take them to the vet for a full check up to be sure they are healthy.
My first reply assumed (and we all know about assuming, right?) was that the rescue group already had the kitten vetted.