The word "ventre" seems to confuse some people. The most reliable dictionaries (Larousse, Wordreference, and Oxford) define it as "stomach". However we must consider that the English "stomach" can mean different things:
- stomach (the organ) : This is not what "ventre" means.
- stomach (the front portion of the trunk of one's body), e.g., I have a flat stomach (or) I have a big stomach.
- stomach (the underpart of an animal or the front of a person, sometimes "belly"), e.g. I stretched out flat on my stomach. My dog loves it when you scratch his belly.
- stomach (general reference to one's gastrointestinal tract). One may say J'ai mal au ventre (I have a stomach ache). The discomfort may be in the literal stomach (l'estomac) or it may be elsewhere.
So the French "ventre" does mean stomach but not in the sense of the stomach organ (l'estomac). If you want to refer to the stomach organ in French, you must say "l'estomac". If you want to say stomach in one of the other meanings shown above, you must use "ventre".
A few sources include the meaning of "abdomen" for "ventre", but be aware there is a separate French word for that: l'abdomen.
Note that in English, stomach is the standard word used. Informal terms include: belly, gut and tummy (which is considered child-speak).
Your answer is accepted, of course, and preferred by US English-speaking adults; our Brit friends may say "a bad stomach" (also accepted). Sadly, there are times when natural English is thrown under the bus so that the back-translation of tummy (that vague area in one's abdomen, also child-speak for "stomach") will return you to "ventre".
We are constantly learning the "real" expressions that people use but they range from standard to quite informal. The underlying issue has been resolved on Duolingo's side by coming to an understanding that "avoir mal au ventre" is expressed (generally) in English as "to have a stomach ache". Of course, there are going to be variations, but the point is that the French does not have to say "l'estomac" in order to be translated as "stomach" in English when using "avoir mal au ventre". Based on actual usage, "mal d'estomac" may be heard, but by far the most common expression in French is "mal au ventre" and that's where our focus should be.
"I'm sick to my stomach" is a very normal thing to hear in US English. However, it can have two meanings. One, more literal, that I'm nauseous. The other, more metaphorical, that I'm very upset, disgusted by something. "I'm sick to my stomach over the situation in the Middle East."
And by the way, what I learned in French (in Québec) for being nauseous is "J'ai mal au cœur." Sounds weird when I translate it (I'm heartsick?).
The idea is that in French, j'ai mal au ventre, but in English it is: I have a stomach ache (or) I have a sick/bad stomach (or) I am sick to my stomach (or) My stomach/belly hurts. These various expressions are all idiomatically correct in different parts of the English-speaking world.