Whats wrong with my kittens back leg?

The kitten isn't actually mine, she's my moms friends kitten. They said the kitten is 4 months old and that she fell of the bed and hurt her leg, even if that was true the kitten is smaller than my aunts 4 WEEK old kittens and still isn't walking like she should. I was able to get the kitten to eat for the first time yesterday and she started moving around in my lap more but if u put her on the floor she just plops over on her side. She moves and stretches out her front paws but nothing much with the back ones. The only major thing she did while laying on the floor was scooted back or kinda jumped back because she was scared of the toy. what could be wrong with her leg? my mom and i were thinking maybe she's just double jointed but the chick swears up and down she's not so what do we do?