腸が痛い: How do you know when your intestines hurt?

Question

I am a foreigner working at a company in Japan. My native language is English but I believe I can converse somewhat moderately well in Japanese. After dinner one evening, I heard the following exchange between two of my coworkers, who are Japanese natives (this may be not the exact wording, but this is the best I can recall):

A: 「うっ…なんか胃が痛い」
B: 「え、腸じゃなくて？」
A: 「うーん…分からない」

On another occasion, I have also had the following exchange with a different Japanese friend who does not work at the same company (again, the wording may not be exact):

Me: 「さっき食べたらちょっと胃が痛くなってきた気がする…とりあえず胃薬飲んでみた」
Friend: 「え？それ胃じゃなくて腸じゃないの？胃薬じゃなくて整腸薬飲めばよかったよ、それ」

As someone who was raised in another country primarily speaking English, I can not once recall ever thinking, "Gee, my intestines hurt." So my question is, under what circumstances would a Japanese person think specifically that their intestines hurt as opposed to their stomach? Or does 腸 include some other part of the body in the abdomen where you would feel pain?

I suppose I could understand if the person were suffering from constipation or diarrhoea for example, but I've never personally regarded either as a "pain in my intestines", nor have I ever recalled experiencing any kind of sensation that I could pinpoint as coming from my intestines, so the idea that "自分の腸が痛い" seems unusually specific to me.

Is this some kind of cultural difference, or am I just weird, born without feeling in my intestines or something?

Edit:

I realize now that I don't recall actually hearing the specific expression "腸が痛い", so I believe my original premise may be flawed. As naruto's answer suggested, both cases were most likely a case of informal "diagnosis", which I admit seems obvious to me now in retrospect.

Answer 1

胃が痛い is common, and somewhat specifically used for so-called epigastric pain. お腹【なか】が痛い is used for pain in any part of the abdominal region.

A person who has right lower quadrant pain (for example, due to appendicitis) usually refers to a physician saying お腹が痛い, 腹痛がある, 下腹部痛がある, etc. At least in standard Japanese, 腸が痛い is not a common phrase to describe symptoms even among those who have chronic bowel diseases.

However, sometimes people try to "diagnose" the cause of abdominal pain. 「それ胃じゃなくて腸じゃないの？」 in that context means "Isn't your pain caused by an intestine trouble?", not "Aren't you feeling pain in your intestine?" For example, one may say this if he is thinking of traveler's diarrhea. Or one may say this when he knows the difference of symptoms of gastric ulcer and food poisoning.

Answer 2

胃が痛い is usually used but I think 腸が痛い isn't very common and we say おなかが痛い than that.

If a person's intestines of health get worse like astriction and diarrhea, we say usually "お腹の調子が悪い"