Started having pencil thin stool. i'm really scared! do i have colon cancer?

It is concerning: You are correct in being concerned, particularly if this is a change. When the stool gets thinned like you are describing, you are concerned that there is something that is obstructing the normal pathway such as a cancer. You should have this worked up by your physician who may have you do tests like colonoscopy or ct scan.

Have it checked: Change in bowel movements can be a sign of colon cancer. It can also be a sign of other non-cancerous conditions of the colon as well, like diverticulitis. If this is a change for you, i would recommend going to a gastroenterologist and having it checked.