Can a CT on abdomen with contrast show colon cancer if there are present symtoms?

Not always. Unless the cancer lesion is fairly large, colon cancer is not commonly seen on a ct scan. Colonoscopy is the best test for colon cancer. Some places have the ability to perform a specialized ct colonography x-ray exam that may see lesions about 1 cm or larger.
It can, if it is. Large enough, but not necessarily. The best test is colonoscopy. At your age, colon cancer is rare, unless you have ulcerative colitis or positive family history.