I get very worried about disease and i'm worried about colon cancer as i get bad constipation and its because i have a bad diet but i get symptoms like sometimes blood quite rarely and pain in anus and earlier it was thin could this be serious ?

Doctor can examine: Young adults under age 25 almost never have bowel cancer. A person cannot tell if he is the rare unlucky one to get cancer at a young age. More importantly, a person absolutely cannot live his life worrying about rare diseases. The best one can do is live a healthy life, see the doctor for evaluation and do a colonoscopy if advised. That means start with a full exam by one's primary care doctor.