Body negativity

I'm wearing my 9 year old sisters size Medium tshirt. And it fits. So why do I look at myself and hate the way I look? My stomach is too big/looks like a pot belly. My legs have cellulite and my butt is too small/flat/saggy. But yet I'm small enough to wear a kids shirt? How does that work??? I think I might have some body issues because everyone tells me I'm skinny but I just don't believe it.