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

Irritable Bowel Syndrome (IBS): Although IBS can present with a variety of symptoms including abdominal pain, bloating, and changes in bowel movements, the presence of occasional diarrhea without constipation and the upper mid gastric pain could suggest this diagnosis. However, the lack of clear alternation between diarrhea and constipation makes it less typical.
Gastroparesis: This condition involves delayed stomach emptying and can cause nausea, vomiting, bloating, and abdominal pain. The symptoms of nausea, bloating, and upper mid gastric pain could be consistent with gastroparesis, especially if there's a history of diabetes or other risk factors.
Small Intestine Bacterial Overgrowth (SIBO): SIBO can cause bloating, gas, abdominal pain, and changes in bowel habits, including diarrhea. The symptoms described could fit SIBO, particularly if there's a history of antibiotic use, bowel surgery, or other predisposing factors.

Peptic Ulcer Disease: Although the patient denies acid reflux, peptic ulcers can cause upper abdominal pain and nausea. The absence of bleeding does not rule out this diagnosis, as many ulcers do not bleed.
Gastric Cancer: This is a critical diagnosis not to miss, especially in older adults or those with risk factors such as a family history of gastric cancer. The symptoms can be nonspecific and include weight loss, abdominal pain, and nausea.
Pancreatitis: Both acute and chronic pancreatitis can present with upper abdominal pain, nausea, and vomiting. The pain can be constant and may radiate to the back.

Celiac Disease: An autoimmune reaction to gluten that can cause abdominal pain, bloating, diarrhea, and malabsorption. It's less common but should be considered, especially with chronic diarrhea and abdominal pain.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic diarrhea, abdominal pain, and weight loss. While the absence of blood in stool makes this less likely, it's still a possibility, especially if other symptoms like fever or significant weight loss are present.
Intestinal Obstruction: This is a rare but serious condition that can cause abdominal pain, nausea, vomiting, and changes in bowel habits. It would typically present more acutely than the scenario described.