Hello!
Thank you for the query.
Left abdominal side is the area where large intestine is located indeed. Thin persons are able to palpate
sigmoid colon in this area (in middle and lower left abdomen). So such symptoms with pain in this area can indicate large intestine problem.
Other possible reasons of
constipation are bad diet (seems yours is fine), chronic bacterial infection, medicines (anti-depressants, Ca-blockers), thyroid diseases, pregnancy.
If you are over the age of 50,
colon cancer should be ruled out at first. In such case,
colonoscopy is the most important test for you.
If you are young person, inflammatory bowels disease can be the reason. In such case I suggest you to start with blood work, abdominal
ultrasound, thyroid tests, pregnancy test and stool tests. In case of blood in the stool, colonoscopy should be done.
Hope this will help.
Regards.