Irritable Bowel Syndrome (IBS): This is the most likely diagnosis given the chronic nature of the symptoms and the absence of abnormal laboratory findings. IBS is a functional gastrointestinal disorder characterized by recurrent abdominal pain and changes in bowel habits, such as diarrhea, constipation, or a mix of both, without any visible signs of damage or disease in the digestive tract.

Functional Dyspepsia: Characterized by persistent or recurrent upper abdominal pain or discomfort that is not exclusively relieved by defecation or associated with the onset of a bowel movement. It's a common condition that can coexist with IBS.
Small Intestine Bacterial Overgrowth (SIBO): A condition where there is an excessive growth of bacteria in the small intestine, leading to bloating, gas, abdominal pain, and changes in bowel habits. It can be a cause of chronic gas and abdominal pain, especially if other causes have been ruled out.
Food Intolerances: Such as lactose intolerance, fructose malabsorption, or gluten sensitivity (without celiac disease), can cause similar symptoms due to the inability to properly digest certain nutrients.

Celiac Disease: An autoimmune disorder where the ingestion of gluten leads to damage in the small intestine. It can present with nonspecific gastrointestinal symptoms, including abdominal pain and gas, and is crucial to diagnose due to its potential for long-term complications if left untreated.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can present with chronic abdominal pain and changes in bowel habits. Although labs may initially be normal, these conditions can lead to significant morbidity if not diagnosed and treated properly.
Gastrointestinal Cancer: Although rare, especially in younger populations, gastrointestinal cancers (like gastric or colon cancer) can present with nonspecific symptoms such as abdominal pain and changes in bowel habits. Early detection is crucial for treatment outcomes.

Eosinophilic Gastroenteritis: A rare condition characterized by the presence of eosinophils in the gastrointestinal tract, leading to inflammation and symptoms like abdominal pain, nausea, and vomiting.
Gastroparesis: A condition where the stomach takes too long to empty its contents, leading to nausea, vomiting, feeling full after eating only a small amount of food, and abdominal bloating.
Intestinal Obstruction: A blockage that prevents the normal flow of the products of digestion, which can cause severe abdominal pain, vomiting, and constipation. Although it often presents acutely, partial or intermittent obstructions might have a more chronic presentation.