I am obese, and had ct, mri, colonoscopy, etc. tests. doc says no anal cancer.i have hemmorhoids.i keep reading online docs can misdiagnose a.c. true?

Why so worried? : Again, why do worried? It sounds like you have already undergone an extensive evaluation. Rectal and colon cancer are very rare at your age. Typically, a flexible sigmoidoscopy is sufficient in your age group. Given your young age, and extensive studies, I think your risk is low. Are there other symptoms or lifestyle choices that make you worry about anal disease?

No: Anal cancer is not considered to be a diagnostic challenge. You have had endoscopy which looks directly at the lining of the colon for any suspicious changes. You have had imaging which looks beyond the surface into the deeper layers of the colon & rectum. A cause for bleeding has been found: hemorrhoids. I wouldn't worry that you have anal cancer. Consider why you might be focusing on this.

High risk?: My guess is that you think you are at high risk because of sexual practices, HIV, or HPV infection or another reason. Even if treated for HIV, risk is about 1.3%/year; small but worrisome. Expert in anal disease could do high resolution anoscopy and cytology to screen for disease not visible with tests you mentioned. Screening is not perfect. If negative, check back with MD periodically.