Perhaps:
His stomach twisted into a knot.

Hmm ... Ist es noch nicht ganz.

Meinst Du, ich kann sagen: "My stomach contracted"? Oder GEHT das nicht??

It's a bit tricky to translate because there's no exact match in English. You say your guts are churning ('dein magen dreht sich um'), but this is not what you wanted to use. I personally think it's sometimes better to concentrate on the feeling rather than the words themselves. So I think you could use "My guts were churning" as well as "I felt queasy" because in both cases it expresses 'ein mulmiges, unangenehmes gefühl in der magengegend'.

Sure, you may say it if that's what you mean. I'm just not sure that it is a common expression in this context.

Sorry: #4 addresses the question in #2.

Thank you both!

Ich würde es mit der "contract-Lösung" versuchen, weil es mir nicht darum geht, daß es ihm schlecht geht, daß ihm übel ist oder so ... Der Magen, der sich zusammenzieht, das ist eine körperliche Reaktion, nicht nur ein mulmiges Gefühl.

Woah, sammy, danke! Das geht? Das trifft gut, was ich meine! ^^

I don't see why not. And you're welcome. :-)

"My stomach was clenching"??? I doubt any native AE speaker would day this. "My guts were churning," I could possibly see, but even that is very odd sounding to me.

I like the expression with "knot" as the most natural sounding to me: My stomach twisted into a knot.

But native AE speakers might also say, "I suddenly felt sick to my stomach" or "I had a sick feeling in my stomach." I also like Kiwiana's "I felt queasy." This could also be modified to something like "Suddenly my stomach was very queasy."

http://www.ellenhenderson.com/excerpt1.htm
Vi's stomach clenched again, and she forced herself to take hold of the situation. “What are you talking about? And is it possible for you to stop with the mysterioso act and just say whatever the hell you want to say?”

http://www.newconceptspublishing.com/thetempl...
Her stomach clenched and she feared she would be ill. She rubbed her hands on a towel and sat on the edge of her bed to await the summons to join the circle ...
 
http://www.frederica.com/writings/spiritual-d...
My heart was pounding, my stomach was clenching, my mouth was dry.

@Sharper: you can't be serious! I doubt any native AE speaker would say this.

I am a native AE speaker and have read this many times: Her stomache clenched, was clenching," etc. There are plenty more examples (thousands), if you care to check.

stomach (sorry

#8 sammy
The verb "to clench" is transitive; it requires a direct object. One can say "I clench my fist", but one cannot say "My stomach was clenching."

sorry, but to clench can also be intransitive.

http://www.msnbc.msn.com/id/22888552/
By Tom Raum
Associated Press
updated 7:42 p.m. PT, Mon., Jan. 28, 2008
WASHINGTON - The state of the union is anxious.
In the eighth and final year of George Bush’s presidency, Americans have stomach-clenching worries about a recession, soaring fuel costs, huge budget deficits, the war in Iraq, fighting in Afghanistan, a showdown with Iran, global terrorism — the list goes on and on.

http://torontopoets.com/poetry/showthread.php...
Unrequited love, which fills you with an unquenchable yearning. There is love impossible where two people are in love and every obstacle has been placed in their way and is never fulfilled. Whatever it's form, i'll take the gut wrenching, heart pounding, palm sweating, head aching, nail biting, stomach clenching love any time over the alternative.

http://www.medhelp.org/forums/gastro/messages...
For instance, sometimes I feel something like my stomach clenching up and I will hear the sound.

I could go on an on, but I won't.

Okay, I stand corrected. It seems, consulting a number of references, that "to clench" has indeed changed from a purely transitive verb to a verb that can be both transitive and intransitive. Apparently, this transition has completely bypassed old fogies like myself. While, still, the intransitive usage of "to clench" makes me clench certain parts of my body, I acknowledge that nowadays it seems to by accepted. -- My apologies to sammy.

Sorry for the late response to this. I will confirm the use of "stomach-clenching" as an adjective (e.g., a stomach-clenching ride). But such usage is really a transitive use--the activity or emotion clenches the stomach. Moreover, "stomach-clenching" is a set phrase in English. But to direct intransitive use like "my stomach clenches" is extremely odd sounding to me, and such a usage is not recognized by the hardcopy 9th edition of Merriam Webster or this on-line American Heritage dictionary: http://www.thefreedictionary.com/clench .

Although a Google search turns up a lot of "my stomach clenched," the stomach clenching reaction that pffft and I have to this usage (and the lack of recognition of such usage in at least some dictionaries) points to it be less than fully acceptable, and perhaps non-standard, if not substandard English.