What Causes Loss Of Appetite And Abdominal Pain And Fever?

My symptoms are loss of appetite, extreme cervical, lower back, and abdominal pain, leg cramps (mostly upper leg), fever, and an internal heart burn that stays within my chest. The cervix pain is most often the worst pain and it is the worst while standing. I went to the doctor and had my blood taken and urnine and they do not know what is wrong with me but confirmed I am not pregnant. What could be wrong?