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.