An MRI or Ultrasound may be more helpfull in this case than a CT scan, as these tests are better at looking at soft tissue, and detecting tumor like masses. You may also want to talk to your doctor about whether or not a colonoscopy may be warrented in your case, it may find something inside the colon that would not otherwise be visible on standard test. If a colonoscopy shows nothing, exploratory surgery may be warrented. Your spleen, is located much higher on the left side, partly under the rib cage, so I would suspect your colon.

