My stomach feels upset-like it's full of knots. (Para. 3)
Meaning: My stomach is making me feel ill. It feels like it is full of tight lumps.
Notice that the expression comes from the idiom "knots in one's stomach", which means "feeling nervous, tense or physically ill".
upset: (1) a. worried, anxious, physically unwell
People get upset when they hear bad news. 聽到壞消息，人們深感不安。
Mrs. Smith was upset when she found out about her husband's telling a lie. 史密斯太太發(fā)現(xiàn)丈夫在撒謊，感到很傷心。
(2) vt. make (someone) worried or physically unwell
Sandy's music upset her father. 桑迪的音樂使她父親感到不安。