The main consideration in this case would be IBD versus infectious colitis

The main consideration in this case would be IBD versus infectious colitis. The absence of travel history, sick contacts, and the chronicity of the illness all point away from infection.
The combination of:
Abdominal pain
Bloody diarrhea
Abdominal X-Ray
localizing the disease to the colon points to a “colitis”.
The “IBD” is most commonly diagnosed in young patients between the ages of 15 and 25 years. There is a second peak in the incidence of IBD (usually crohn´s disease) between the ages of 60 and 70 years. The IBD may present with a low grade fever. Anemia may be present, either due to iron deficiency from chronic GI blood loss, or anemia of chronic disease. Patients with IBD may also report fatigue and weight loss.
Ulcerative colitis (UC) usually presents with grossly bloody stool, and also involves only the large bowel, whereas Crohn´s disease may affect any portion of the GI tract، typically the colon and terminal ileum. UC always begins in the rectum and proceeds proximally in a continuous pattern; disease is limited to the colon. UC is characterized by diarrhea and typically leads to bowel obstruction. The diagnose usually is confirmed after colonoscopy with biopsy of the affected segments of bowel and histologic examination. In UC inflammation will be limited to the mucosa and submucosa, whereas in Crohn´s disease, the inflammation will be transmural (throughout all layers of the bowel).