The reason for the pain will depend on its location. I am enumerating the causes for both males and females as you have not registered your gender.
Lower right abdomen pain is commonly due to appendicitis. The pain, can at times come and go and not give the classic picture. Crohn’s disease, diverticulitis, inflammatory bowel disease, infection in kidney, kidney stones, constipation, ovary involvement in women (cysts, hemorrhage, tumor, torsion—torsion is a medical emergency and a long standing pain is not due to this), ectopic pregnancy in women, mesentry (a thin tissue that carries the blood, nerves and lymphatics for the intestine) involvement low blood supply, enlarged lymphnodes, and an abscess in rectus abdominis muscle or psoas muscle (these two are unlikely is there is no localized patch of heat or lump) can all cause lower right abdomen pain. It can also be due to inguinal hernia. Other than that at times pelvic inflammatory disease causes a lower abdominal pain along with a pelvic pain.
Right upper abdominal pain can be due to cholecystitis (inflammation of gall bladder), stones in gall bladder, abscess under the diaphragm, liver problems or due to duodenal ulcer. Right flank pain is generally due to kidney stones.
Pain and discomfort on the lower left side of abdomen just below and to the left of navel can be due to several causes such as Crohn’s disease, colitis, diverticulitis, infection in kidney, kidney stones, adhesions, polycystic ovary in females, IBS, constipation, and an abscess in rectus abdominis muscle or psoas muscle. It could also be a periumbilical hernia that has got trapped resulting in small bowel obstruction or mesenteric ischemia. There is a possibility that there is a stone in the urinary system. This also causes pain that can be felt in groin and hip area.
Do discuss this with your doctor and get yourself examined. Take care!