If your kitten's head is leaning back and its forelimbs are paddling, it may be because it is playing or trying to catch something. This behavior is usually because the kitten wants to pull something closer to herself or push something away. If this behavior happens occasionally, it may just be a playful gesture, so don't worry too much.
However, if this behavior occurs frequently and is accompanied by other symptoms, such as vomiting, incontinence, convulsions and so on. Then maybe the kitten is ill and needs to see a doctor in time. In this case, it is suggested that you take the kitten to the vet as soon as possible to diagnose and treat the disease in time.