Idiomatic expression about concern or worries with "stomach"
You could use the expression:
pit of one's stomach:
Fig. the middle of one's stomach; the location of a "visceral response." “I got a strange feeling in the pit of my stomach when they told me the bad news.”
(McGraw-Hill Dictionary of American Idioms and Phrasal Verbs)
The adverse stimulus can be described as stomach-churning:
ADJECTIVE If you describe something as stomach-churning, you mean that it is so unpleasant that it makes you feel physically sick.
- The stench from rotting food is stomach-churning.
- ...that rush of stomach-churning fear at the sound of a mortar exploding nearby.
As with Cobuild's second example above, example sentences from Longman show the at least partially metaphorical broadening, certainly involving dread:
• For much of the past week I have had this dreadful, stomach-churning feeling of imminent disaster.
• At the end of every set of three levels a stomach-churning guardian awaits.
while Macmillan highlights the dread-inspiring nature:
- stomach-churning: making you feel very frightened, nervous, or ill
“To worry oneself sick [about or over something]” is one – the word “sick” implies a state of nausea or illness.
“I worried myself sick about the exam results.”
“To be sick to the stomach of/with something” = to be irritated and annoyed by something but this is old-fashioned and is now reduced to “to be sick of/with [a repeated action]”
“I am sick to the stomach of your constantly complaining about everything I do!” (old-fashioned)
“I am sick to the stomach with your constant complaints about everything I do!” (old-fashioned)
“I am sick of your constantly complaining about everything I do!” (Current)
“I am sick of your constant complaints about everything I do!” (Current)
Have one's stomach (tied) in knots
used to say that a person has an unpleasant and tight feeling in the stomach, usually from nervousness
Her stomach was (tied) in knots as she awaited the start of competition.
Cambridge Dictionary