ありがとう[emoji]

すごく たいせつ ね。

Thank you for Jiayin's question.

In a sense that "onaka suita" is a casual form, you could say that it is following a different grammatical rule.

"Onaka" means stomach, but "suita" is the past tense of the verb "suku", which means to go empty or to become less crowded.  

For how the formal sentence changes to the informal sentence, you are abbreviating the "ga" particle here, and you are using the basic verb form of "suku" and its past tense "suita" instead of the polite form "sukimasu" and the polite past tense "sukimashita".

How to change from basic verb form to the polite "masu" form, please see the following.

1) For ru-verbs: Remove "ru"  Ex: Miru (to see, basic) →  Mi + masu → Mimasu (to see, polite)

2) For u-verbs: Remove "u" and change to "I" Ex: Kau (to buy, basic) → Kai + masu → Kaimasu (to buy, polite) 

Exceptions: suru (to do) → shimasu, kuru (to come) → kimasu

You can see that "suku" is an example of "u" verb. Therefore, for the polite masu form is, remove "u", add "I"+masu, which results in "sukimasu" and the past becomes "sukimashita."

Hope that helps!