Viral Gastroenteritis: This is the most likely diagnosis given the acute onset of diarrhea, frequent bowel movements, and dark liquid stool. The fact that Imodium is not helping also points towards an infectious cause. The recent consumption of a leftover burger could be a potential source of infection, even if the patient has eaten leftovers before without issues.

Bacterial Gastroenteritis: Similar to viral gastroenteritis but caused by bacteria. The dark stool could suggest the presence of blood, which is more common in bacterial infections. However, the absence of other symptoms like fever or abdominal pain makes this less likely.
Food Poisoning: Given the history of eating a leftover burger, food poisoning is a plausible diagnosis. The symptoms align with common foodborne illnesses, although the lack of fever or vomiting makes it slightly less likely.
Medication-induced Diarrhea: Although not mentioned, if the patient has recently started any new medications, this could be a cause of his symptoms. Certain antibiotics, laxatives, and other medications can cause diarrhea.

Inflammatory Bowel Disease (IBD) Flare: While less likely given the acute onset, it's crucial not to miss a flare of IBD (Crohn's disease or ulcerative colitis), as it requires specific management. The dark stool could indicate bleeding, which is a red flag.
Clostridioides difficile (C. diff) Infection: Especially if the patient has been exposed to antibiotics recently, C. diff infection is a critical diagnosis not to miss due to its potential severity and the need for specific treatment.
Intestinal Ischemia: Although rare and more common in older adults with vascular disease, intestinal ischemia can present with sudden onset of diarrhea and abdominal pain. It's a medical emergency.

Giardiasis: An intestinal infection caused by the parasite Giardia lamblia. It's less common but could be considered if the patient has been camping, drinking untreated water, or has been exposed to someone with the infection.
Lactose Intolerance or Other Malabsorption Syndromes: If the patient has recently increased his intake of lactose-containing products or has another malabsorption issue, this could lead to similar symptoms. However, the dark stool and high frequency of bowel movements make this less likely.
Thyroid Disorders: Hyperthyroidism can cause diarrhea among other symptoms. This would be an unusual presentation but should be considered if other symptoms or signs of thyroid disease are present.