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. 听到坏消息，人们深感不安。
Mrs. Smith was upset when she found out about her husband's telling a lie. 史密斯太太发现丈夫在撒谎，感到很伤心。
(2) vt. make (someone) worried or physically unwell
Sandy's music upset her father. 桑迪的音乐使她父亲感到不安。