An ultrasound would help to evaluate the spleen, but you should ask your GP about an abdominal CT which will provide much more detailed information about all the internal organs. If there is suspicion of a colonic process (eg. cramping, diarrhea, black stool, etc) you should also consider a colonoscopy. This provides important information about what is going on inside the colon, which is not well visualized on CT or ultraound. Colonoscopy is done by Gastroenterologists.
I would advise you get a CT scan. That will provide the answer.