First, regarding your stomach 'bulging' concerns, don't fret because unless your abdomen is -really-swollen then it is probably nothing. Probably you just begin to notice things or question things only because you're worried in the first place. So don't worry yourself about how your abdomen looks, unless you're 100% sure it looks different than it has before...
Otherwise, my only other question would be 'What other symptoms do you have?' Is there pain/cramps/aches of any kind associated with or relieved by your bowel movements or otherwise? Or have you been nauseous or vomited at all? If not, then don't be worried about constipation...at the worst it could probably be irritable bowel syndrome, although there is usually cramps/pain associated with that.
Do not worry about cancer at all. Unless you have family history of colorectal cancer, in which case you might be a little more concerned, then you shouldn't worry at all. If you did have colorectal cancer you would probably be seeing drastic changes, such as blood in your stools, diarrhea or longer and narrower stools, along with weight loss and possibly mild pain.
Hope I can help, and do please elaborate...many people at some time or another become worried of cancer or other diseases of the intestines, especially because altered bowel habits seem to be more noticable and worrisome than other types of symptoms (like a headache or something)
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