Muscle strain: This is a common issue following physical exertion, such as cardio light testing. The strain could occur in the abdominal muscles, leading to pain in the left lower abdomen.

Constipation or gastrointestinal issues: Physical activity can sometimes exacerbate gastrointestinal symptoms or cause constipation, leading to abdominal pain.
Hernia: Although less common, a hernia could become symptomatic or worsen with physical exertion, causing pain in the lower abdomen.
Ovarian cysts or other gynecological issues (in females): These can cause lower abdominal pain and might be exacerbated by physical activity.

Appendicitis: Although the pain is typically in the right lower quadrant, it's crucial not to miss this diagnosis, as appendicitis can present atypically, especially in early stages or in certain populations.
Diverticulitis: Inflammation of the diverticula in the colon can cause severe abdominal pain, usually in the left lower quadrant, and requires prompt medical attention.
Ectopic pregnancy (in females): This is a life-threatening condition that can cause abdominal pain and must be considered, especially if the patient is of childbearing age and has a missed period.
Ruptured ovarian cyst (in females): This can cause severe, sudden pain and is a medical emergency.

Intussusception: A condition where a part of the intestine slides into an adjacent part, which can cause abdominal pain, but is more common in children.
Inflammatory bowel disease (IBD) flare: Conditions like Crohn's disease or ulcerative colitis can cause abdominal pain and might be exacerbated by physical activity, though they are less likely to be directly caused by cardio light testing.
Abdominal aortic aneurysm: Although extremely rare in younger individuals, an abdominal aortic aneurysm can cause abdominal pain and is a medical emergency.