Gastroesophageal Reflux Disease (GERD): The presence of nausea, intermittent constipation, and noting bits of undigested food in stool could suggest GERD, especially if the patient experiences symptoms like heartburn or regurgitation, which are not explicitly mentioned but could be inferred.

Irritable Bowel Syndrome (IBS): The combination of nausea, constipation, and loose stools, along with the absence of alarming symptoms like significant weight loss, blood in stool, or fever, makes IBS a plausible diagnosis.
Functional Dyspepsia: This condition, characterized by persistent or recurrent upper abdominal discomfort or pain, could explain the nausea and the presence of undigested food in the stool if the patient has difficulty digesting food properly.
Early Pregnancy: Although the patient is not currently preventing pregnancy, early pregnancy could cause nausea without vomiting, and the gastrointestinal symptoms could be attributed to hormonal changes.

Peptic Ulcer Disease: Despite the absence of NSAID use or a history of stomach ulcers, peptic ulcer disease could still be a consideration, especially if the darker stools are indicative of melena (blood in the stool), suggesting gastrointestinal bleeding.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis could present with a variety of gastrointestinal symptoms, including changes in bowel habits and blood in the stool, and are critical to diagnose early to prevent complications.
Gastrointestinal Obstruction: Although less likely given the absence of severe abdominal pain or absolute constipation, a partial obstruction could explain the undigested food in the stool and should be considered, especially if the patient's condition worsens.

Gastroparesis: A condition characterized by delayed stomach emptying, which could explain the nausea, feeling of undigested food, and changes in bowel habits. It is less common but should be considered, especially if other diagnoses are ruled out.
Small Intestine Bacterial Overgrowth (SIBO): This condition, where bacteria overgrow in the small intestine, can cause a variety of gastrointestinal symptoms, including bloating, abdominal pain, diarrhea, and malabsorption, which might fit some of the patient's symptoms.
Celiac Disease: An autoimmune disorder where the ingestion of gluten leads to damage in the small intestine, causing malabsorption and various gastrointestinal symptoms. It's a less common condition but important to diagnose due to its specific treatment (gluten-free diet).