You mentioned that your stools occasionally change shape. If there were something there, like a tumor, they would most likely constantly be narrow or flat. Go to the doctor just for reassurance, but at your age and without any other alarm symptoms (weight loss, fatigue, etc.), I would say you're fine.
Anyone? I am quite worried about this.
I am 20 years old, thinly built (5'8 and 110lbs), and get lots of activity (I'm a long distance runner). I also have no family history of cancer or colon polyps.
Medical books suggest that you should seek advice if your stools change in shape. By how much/to what degree/for how long is not stated!!
You don't state your age but from the age of ~40 upwards you are more prone to develop the more sinister G.I. diseases.
A colonoscopy is probably the test that can provide maximum reassurance. My tendency would always be to seek this test since one of my relatives (aged ~70) ignored initial symptons and eventually succumbed to colon cancer so my motto would be "always err on the side of caution".
Good luck
Morecambe