Why would someone have drowsiness, listlessness, abdominal pain, and anorexia?

Too Broad. There are just too many different diseases that can cause these symptoms; with the limited information, it's impossible to narrow it down. However, it is easy to connect all these symptoms. It is common to have anorexia associated with abdominal pain. Drowsiness can occur with any disease that depresses the central nervous system, ie drugs, infections, cancers. I advise a formal medical eval.