First, unless you have a rare genetic disorder (which I believe may be hereditary), you're too young for colon cancer, since it takes years for that to develop.
As for you not going, the obvious question here is your diet: are you getting enough fiber? (I generally follow the "at least 40g per day" rule.)
The fact is that there are many, many things this could be, most of them not as serious as you think. I'm not going to go into what you should be watching out for because if you're a high-anxiety individual (speaking as one myself) you'll probably start seeing it, regardless whether there's something there or not. I will say that the color of what comes out and how it acts is of importance: regularly it should be soft but formed, like a paste, and about the color of a cardboard box. Darker and harder probably means that you haven't been eating enough fiber. Other colors you should watch out for are black/tarry (possible bleeding), gray or clay colored (possible bile issues, or side effect of Aluminum hydroxide), orange (common if you've been eating lots of spinach-like foods), and green (usually related to purple food coloring and fast colonic transit time). Also you should notice whether it floats or doesn't and how fast it sinks (as far as you can tell). Finally also keep a close eye on your diet and the food you eat.
Now that we have that out of the way, first and foremost, go to the doctor's office. They'll ask you questions, maybe run some tests, and take it from there. They'll probably start by asking you what your diet is like, and if there's not enough fiber, they'll recommend you start, maybe give you something to move things along. If you already eat a fair amount of fiber (say, you're a vegetarian), then they may look for other causes of the constipation.
Good luck.
Dear Guitarzar your Dr.should prescribe to you some Lactulose it's a liquid product that you can drink and it won't hurt u at all and it's not addictif.You can buy it the pharmacy without prescription.Please let me kno hope u are doing well pogo