The problem with abdominal pain is it can be anything from a pulled muscle, to intestinal gas, to a gastric ulcer, to a bowel obstruction, to kidney stones, to gall bladder problems, to infections, to spleen or pancreatic problems, to ruptures, and more.
Abdominal pain radiating to the back, weakness, and a low grade temperature are sort of red flags. People don't get a temperature if they have gas or if they pull a muscle. The constipation could be from a blocked intestine.