This is a common problem with kitties and puppies that are removed from their mothers too soon especially in the runts in the litter. They don't get enough suckling time and aren't prepared for separation from Mom. If they can find skin on you they will suckle. They also suckle on toys or fabric like babies sucking on a silky edge of a blanket. We have a 2 yr. old dog that was the runt that still sicks on a toy just before sleeping.