What could be the possible cause of constant stomach pain, on and off fever, chills at night, and vomiting?

See a doctor ASAP: This sounds like an infection of some sort inside your abdomen -- things like appendicitis, gall bladder infection, diverticulitis, or if you are sexually active and at risk for STD, pelvic inflammatory disease, a common complication of gonorrhea or chlamydia. Several other possibilities as well. These are serious symptoms; you need to be examined professionally ASAP. Good luck.