The most likely diagnosis in this patient is "Irritable bowel syndrome (IBS)."

IBS is a gastrointestinal condition characterized by abdominal pain or cramps, as well as acute episodes of diarrhea and/or constipation that is often triggered by psychological stressors such as depression or anxiety, or a preceding gastrointestinal infection. IBS is more common in young females than other demographics, and often can be co-morbid with clinical depression or anxiety disorders. Its pathophysiology has not yet been fully characterized. There is no definitive diagnostic test to confirm a diagnosis of IBS, but the lack of blood or mucus in the stool, as well as the lack of a fever or elevated white blood cell count all are consistent with a diagnosis of IBS. Further, in this patient, given that she is a young female whose symptoms for this episode and prior episodes coincided with psychological stressors and resolved with bowel movements and resolution of her stressors, and who also had no fever, elevated white blood cell count, vital sign abnormalities, or concerning historical details (foreign travel, dietary changes, sick contacts), IBS is the most likely diagnosis.

Ulcerative colitis would be more likely to present with recurrent episodes of bloody and/or mucoid diarrhea, as opposed to non-bloody, non-mucoid diarrhea. Further, given this patient's relationship of symptoms to immediate psychological stressors, her symptoms are more consistent with IBS than ulcerative colitis.

Colon cancer would be an unlikely diagnosis in a young, otherwise healthy patient with no family history of colon cancer. The fact that she is experiencing non-bloody, non-mucoid diarrhea does not in and of itself provide evidence for colon cancer. Colon cancer may present with occult blood in the stool, melanotic stools, or blood streaked stools, but would be highly, highly unlikely in a healthy young patient.

Gluten intolerance and lactose intolerance are reasonable thoughts when evaluating a young, otherwise healthy patient with non-bloody, non-mucoid diarrhea. Lactose intolerance would likely present at a younger age though, and the patient's diet reportedly has not changed prior to her diarrhea episodes, which would argue against a dietary etiology of her diarrhea. Further, given the close relationship of her symptoms to acute psychological stressors, IBS is the most fitting diagnosis.

Intestinal atresia is a malformation resulting in narrowing or absence of a portion of the intestine. Duodenal atresia is the most common type, followed by ileal atresia. Hirschsprung's disease is an issue of innervation in the large intestine that can result in narrowing due to contraction, but there is no structural malformation in the bowel itself. Malrotation and volvulus are often seen together when a part of the intestine does not anchor or turn correctly during formation (malrotation) and then becomes twisted around itself (volvulus), resulting in constriction and loss of function.

While an infant with Hirshsprung's disease may have vomiting and bloody stool, the most common sign is failure to pass meconium within 48 hours of delivery. Hirschsprung's disease, which is an absence of innervation to the large intestine, which results in narrowing and constriction of one part of the bowel and dilation of the preceding segment, can be diagnosed by biopsy of the distally narrowed segment of the bowel.

Intussusception occurs when part of the intestine folds into another section of intestine, much like a telescope. This results in sharp, crampy, or colicky abdominal pain, vomit of bile, and bloody red "currant jelly" stool. Black stool and "coffee ground" emesis are both symptoms of upper gastrointestinal bleeding (stomach, generally), while projectile vomiting may be associated with pyloric stenosis.

The most common location for diverticulitis is the sigmoid colon. This area generally has increased pressure as compared to the rest of the large intestine and is especially vulnerable to weakness in the muscle layers of the colon wall.

While individuals with Celiac disease can certainly have gastritis, fistulas, and ulceration due to inflammation, the primary effect of Celiac disease is villous atrophy in the small intestine. Villi become blunted, leading to loss of ability to absorb nutrients, including minerals and fat-soluble vitamins.

While Crohn's disease can affect any part of the alimentary canal, the small intestine, particularly the terminal ileum, is the most common site of serosal inflammation.

The characteristic histological alteration seen in ulcerative colitis is the crypt abscess, in which inflammation causes loss of goblet cells due to neutrophilic exudate in glandular lumens. Granulomas, thickened mucosa, and skip lesions are all seen in Crohn's disease.

In congenital hypertrophic pyloric stenosis, the smooth muscle of the pylorus becomes thickened, decreasing the lumen size of the pylorus. This prevents food from passing out of the stomach and into the small intestine, resulting in vomiting which is often projectile and tends to be non-bilious. Intussusception may show vomiting, but the more obvious sign is bloody "currant jelly" stools. Infant gastroesophageal reflux disease (GERD) may result in frequent spitting up, but projectile vomiting is rare. Diverticulitis is generally a condition of the elderly and tends to present as abdominal pain with alterations in bowel function and mild fever.

Full-thickness (transmural) ulceration of the bowel wall occurs in Crohn's disease. Bowels may also show thickened walls, serosal adhesions, and loss of the regular folds.

In ulcerative colitis, ulceration is restricted to the gut mucosa. Celiac disease results in blunting of intestinal villi, but does not cause ulceration in any form. Irritable bowel syndrome, or IBS, is considered a functional disease as it results in no known pathological tissue changes.