Hi all. I understand the function of んです as a marker of an explanation when it stands alone. What I don't understand is why んです would ever be needed in a sentence with から or んで. For example, what's the difference in meaning between: 食べすぎたから、おなかがいたいです。 vs. 食べすぎたから、おなかがいたいんです。Do they not both just mean 'My stomach hurts because I ate too much?'. TY in advance.