Functional Dyspepsia: This condition is characterized by recurring signs and symptoms of indigestion with no obvious cause, aligning with the described symptoms of bloating, nausea, burping, and upper abdominal pain. The absence of acid reflux and the nature of the pain (constant, achy) support this diagnosis.

Irritable Bowel Syndrome (IBS): Given the occasional diarrhea and symptoms triggered by specific foods (soda, spicy, or greasy foods), IBS is a plausible diagnosis. IBS can present with abdominal pain, bloating, and changes in bowel habits, which overlap with the symptoms described.
Gastroesophageal Reflux Disease (GERD): Although the patient mentions reflux when eating certain foods, the absence of frequent or severe acid reflux symptoms might suggest a milder form of GERD or an atypical presentation. However, the symptom of reflux, especially when consuming specific types of food, warrants consideration of GERD.

Peptic Ulcer Disease: It's crucial not to miss peptic ulcers, as they can lead to severe complications like bleeding or perforation. The patient's symptoms of upper abdominal pain and the potential for NSAID use (not mentioned but a common cause) make it essential to consider this diagnosis.
Gastric Cancer: Although rare, gastric cancer can present with nonspecific symptoms like abdominal pain, bloating, and changes in appetite. Given the potential severity of this condition, it should be considered, especially in older patients or those with risk factors (e.g., family history, Helicobacter pylori infection).
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can present with abdominal pain, changes in bowel habits, and other systemic symptoms. While less likely given the description, IBD should be considered due to its potential for significant morbidity.

Gastroparesis: A condition characterized by delayed gastric emptying, which can cause nausea, vomiting, bloating, and abdominal pain. It's less common but should be considered, especially if symptoms persist or worsen over time.
Small Intestine Bacterial Overgrowth (SIBO): This condition involves an overgrowth of bacteria in the small intestine, leading to bloating, gas, abdominal pain, and changes in bowel habits. It's a less common diagnosis but could be considered if other diagnoses are ruled out and symptoms persist.