Re: pain for 4 weeks
I would see another doctor. An anal fissure is simply a little tear in the sensitive skin around the anus - think of it as a paper cut down there. I have never heard of an anal fissure causing abdominal pain nor fatigue or making someone feel nauseous. However, then can bleed somewhat, but again, I have never heard of them causing a large amount of blood. What they do cause are some pretty excruciating pain when you go to the bathroom because you are basically ripping open these little tears when you pass stool. The amount of blood that they can cause is not usually that much and is mostly little marks on the toilet paper.
I am certainly not saying that you have colorectal cancer, but I would definately go and see another doctor. Any doctor who attributes the symptoms you have to an anal fissure just isn't doing his/her job. Here is what webmd says about anal fissure symptoms.
"Anal Fissure - Symptoms
Most anal fissures cause:
A sharp, stinging or burning pain during a bowel movement. Pain from a fissure may be quite severe. It can be brief or last for several hours after a bowel movement.
Bleeding, often a small spot of bright red blood on toilet tissue. Tell your health professional if you have any bleeding with a bowel movement. Blood mixed with stool usually is a sign of some other condition, possibly inflammatory bowel disease (IBD) or colon cancer.
An anal fissure can be a painless wound that won't heal and that bleeds intermittently but causes no other symptoms"
The blood in the stool is not a good symptom so I would go and see another doctor right away. The symptoms that you describe can not be attributed to an anal fissure.