Department of Medicine is usually Internal medicine If other superspecialization branches are available in the hospital If not, then even superspecialization comes under Dept of Medicine Like Cardiology, Gastroenterology, Nephrology etc. Abdominal pain can be medical or surgical, you can go to anyone first

Medicine deptt looks after all the ailments like fever, blood pressure, diabetes, pain abdomen, diarrhea, vomiting, headache. You can go to them for pain abdomen Good luck

General medicine or internal medicine as it is called, they are trained to diagnose anything, from minor to major problems. You can definitely go to the medicine department and they will look into the problem and diagnose it. If at all it is minor problem they will treat you with medication, if it is something for example appendicitis or gastric ulcer then might refer you to the surgical department for further treatment.