Narrow stools that happen on a rare basis are probably not an evidence of a serious underlying problem. However in some cases, narrow stools – particularly if pencil thin may be an indication of narrowing. Or obstruction of the colon due to colon cancer. In addition, irritable bowel syndrome may cause irregular stools that alternate between normal and either constipation or diarrhea. Such stools might be narrower than usual.
I believe they can mean a lot of things. The best way to determine if its colon cancer is to see a doctor. I know, you probably don't want to or you would have done that instead of posting here. You can also get yourself an FOB test and see if you have hidden blood in your stool. If you do, you need to see a doctor. The test I use comes from colonhealthcheck.com and I highly recommend them.
Most symptoms for colon cancer don't show up until you already have an advanced case. Blood in your stool is sometimes the only advanced symptom, so if you catch it quick you should be fine. Check yourself out!