Irritable Bowel Syndrome (IBS): The symptoms of diarrhea starting after consuming junk food, a history of gas during stressful periods (10&12 exams), and the absence of any findings in the stool test 20 days after the initial diarrhea, point towards IBS. The condition is characterized by recurrent abdominal pain associated with a change in bowel habits, which can be triggered by dietary factors, stress, or other non-pathological stimuli.

Food Intolerance: This could be a reaction to a specific component in the junk food, such as lactose, gluten, or high-fructose corn syrup, leading to gastrointestinal symptoms like diarrhea.
Infectious Gastroenteritis: Although the stool test was negative 20 days post-diarrhea, it's possible that the infection was viral or that the test was done too late to detect the causative agent.
Functional Dyspepsia: This condition involves upper abdominal symptoms such as bloating, early satiety, or discomfort, which can sometimes be associated with lower gastrointestinal symptoms like diarrhea, especially if the junk food triggered an exacerbation.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can present with diarrhea and may not always have positive findings on initial stool tests. Missing these diagnoses could lead to significant morbidity.
Celiac Disease: An autoimmune reaction to gluten, which can cause chronic diarrhea, abdominal pain, and malabsorption. It's crucial to consider this diagnosis to avoid long-term complications.
Giardiasis: A parasitic infection that can cause prolonged diarrhea. Although less common, it's a diagnosis that should not be missed due to its potential for causing malabsorption and weight loss.

Microscopic Colitis: A condition characterized by chronic, watery diarrhea, which can be caused by inflammatory changes in the colon that are only visible under a microscope.
Bile Acid Diarrhea: This condition occurs when the body produces too much bile acid or the bile acids are not properly absorbed, leading to chronic diarrhea.
Small Intestine Bacterial Overgrowth (SIBO): An excessive growth of bacteria in the small intestine, which can lead to bloating, gas, and diarrhea after consuming certain foods.