In my own fan theory, Nala and Scar did have a cub. It wasn't what Nala had wanted, but many of the lionesses were getting sick; and she had to keep the pride alive somehow. Thus Scarla was born. Scar was angry because he wanted to produce a strong king, which lead to Scar disowning her.
Although it had been months since Scar's death, Scarla still didn't feel like she belonged to a family she wasn't even intended to be in
Simba: Scarla? Are you Okay?
Scarla: What do you think!
Simba: Listen, I know the other cubs taunt you. They push you to your limit, but you shouldn't run off like that.
Scarla: Why do you even care? Why don't you attend to your perfect daughter.
Simba: Scarla...I am your Father too, and even though blood wise you aren't mine, I still care for you as my own.
Scarla: Yeah Right! Get over yourself. Just leave me alone.
Simba wasn't able to get through to her, and thought maybe she needed some time alone. So Simba took off on a walk while Scarla went home.
Scarla(C)Me
Render(C)Me
Simba(C) Disney
Original Image(C)Disney