Hi, many thanks for the query!
Yes, your picture exactly fits in IBS (irritable bowel syndrome
One of the most important causes of IBS is "psychological stress", so at present I can see the stress in your mind in the form of "worry if you have colon polyp" and its quite natural because of the trouble you are facing, so even if it is un-necessary, I would suggest you to get a colonoscopy
done, not as a diagnostic tool but as a therapeutic tool to calm down your stress, once it is normal.
Rule out "Lactose intolerance"- most common differential diagnosis, coeliac
disease, parasitic infestations.
Do stool microscopy & culture.
Treatment for IBS is aimed at reducing psychological stress, by meditation
etc. avoid milk & milk products, eggs, if its lactose intolerance.
For belly size, its only EXERCISE that can help, dont decrease your diet for that as it will cause dietary deficiencies of various important nutrients.
Wish you a good health. Take care.