I have had bad pains in my stomach when i lay flat on my belly or on my left side. why is that?

Doctor to evaluate: If one has some uncomfortable tummy pain, and is normally in good health, plus is not pregnant, the possibilities include: appendicitis (usually lower r tummy), diverticulitis (usually l tummy), kidney stone (very painful), constipation (crampy on l), tumors, intestinal infections, etc... A person with more than brief, mild pain needs a doctor's evaluation to be sure it is not something serious.