No, it's not "nearly always" a sign of colon cancer.
Your episode a month ago, with the sensations on the left side of your abdomen (lower left?), would have me thinking diverticulosis. Many people have this condition and never have any symptoms at all. If it becomes very severe or infected, that is diverticulitis, which can lead to other problems.
If that is the case, you are doing exactly the right thing. But since the stool is still small, try to increase the amount of fiber (hopefully from some dietary sources - fresh veggies and fruits). Also, I find it helpful to decrease the amount of no fiber food I am eating (primarily white flour products). Drinking plenty of water also helps as does eating a good yogurt (like the variety from the health food store) regularly.
The sensations on my left side sort of move around. There is one that pretty much stays a couple inches below and to the left of my navel and then some others than can move around as far up as just under the bottom of my left ribcage.
I know a few people who have divirticulitis or the prerunner to that, and they seem to have lots of pain, diareah, naseua, etc. I have none of that. No pain, no diareah no feeling of sickness.
DiverticuLITIS does involved pain and other symptoms like diarrhea. DiverticuLOSIS does not, and could include the sorts of symptoms you describe. My grandmother described her's as a fluttering sensation.
In any event, it would probably be a good idea to consult your doctor or GI for a possible colonoscopy to set your mind at ease about what is going on.