Rectal bleeding, dre found no evidence of piles but stool was dark. 30 y/o. need colonoscopy. cancer?????

Likely not: There are many reasons for rectal bleeding, including internal hemorrhoid, inflammatory bowel disease and diverticulitis, among others. Colon cancer is certainly a reason, but at your age, unless you have a huge family history of colon cancer, it is way down on the list. You should get a colonoscopy for sure, but don't go into it with the worst case assumption