Irritable Bowel Syndrome (IBS): The patient's symptoms of abdominal discomfort, heartburn, and recent changes in eating habits (eating a lot and late at night) without alterations in bowel movements (denies constipation, diarrhea) could suggest IBS. The absence of masses and significant pain also points towards a functional rather than a structural cause.

Gastroesophageal Reflux Disease (GERD): The patient's complaint of heartburn, especially after eating a lot and late at night, is a common symptom of GERD. Although GERD primarily affects the upper GI tract, it can sometimes cause referred pain to the lower abdomen.
Diverticulitis: Although less common in younger patients, diverticulitis can present with left lower quadrant abdominal pain. The absence of fever, significant pain, or changes in bowel habits makes this less likely but still a consideration.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause chronic abdominal pain, changes in bowel habits, and sometimes heartburn due to gastroesophageal reflux. However, the patient's denial of significant changes in bowel movements and the absence of other systemic symptoms (like weight loss or fever) make this less likely.

Appendicitis: Although the pain is on the left side, which is atypical for appendicitis (usually right lower quadrant), it's crucial to consider appendicitis due to its potential for severe complications if missed. The patient's mild pain and lack of significant findings on palpation decrease the likelihood but do not rule it out entirely.
Ectopic Pregnancy (if sexually active with a female partner): Though extremely rare in males, if the patient has a female partner who could be pregnant, ectopic pregnancy should be considered in the differential for abdominal pain, especially if the partner is of childbearing age and the pain is severe.
Intestinal Obstruction: This condition can present with abdominal pain, vomiting, and changes in bowel habits. The patient's denial of significant vomiting or obstipation (inability to pass gas or have bowel movements) makes this less likely, but it remains a critical diagnosis not to miss due to its potential for severe complications.

Intussusception: A condition where a part of the intestine telescopes into another part, which can cause abdominal pain, vomiting, and sometimes blood in stool. It's rare in adults and usually associated with a lead point like a tumor.
Mesenteric Ischemia: This condition involves insufficient blood flow to the intestines and can cause severe abdominal pain out of proportion to the physical examination findings. It's more common in older patients or those with significant vascular disease.
Ovarian Torsion (if applicable): Again, this would be relevant if discussing a female patient, but it's included here for completeness as it's a rare but critical diagnosis in the right clinical context.