My chin looks like there's a ball in it. How can I fix it? (Photo)

Like in the middle it's a balled up circle. Im a skinny person but my face is a bit chubby and I'm a 20year old female. This is how it looks like, Photos down below :1st and 2nd is me purposely straining my mentalis muscle for an example. And also I noticed that when I'm eating or drinking my chin is not relaxed like it then makes my chin look more weird and obvious that I have a ball look in my chin so what's going on in my chin, what caused it and how can i fix it ?