Gastroenteritis: This condition, often caused by viral or bacterial infections, can lead to diarrhea, fatigue, and dizziness due to dehydration and electrolyte imbalance. Muscle tightness could be secondary to dehydration and electrolyte disturbances.

Irritable Bowel Syndrome (IBS): While IBS doesn't typically cause dizziness, it can lead to chronic diarrhea and fatigue. Muscle tightness might be related to stress or the abdominal pain associated with IBS.
Hypokalemia: Low potassium levels can cause muscle weakness, fatigue, and arrhythmias leading to dizziness. Diarrhea can be a cause of hypokalemia.
Dehydration: Severe dehydration from any cause, including diarrhea, can lead to fatigue, dizziness, and muscle cramps or tightness due to electrolyte imbalances.

Botulism: Although rare, botulism can cause muscle weakness, fatigue, and dizziness. Gastrointestinal symptoms like diarrhea can occur, especially in infant botulism.
Food Poisoning (e.g., from Salmonella, E. coli): Certain types of food poisoning can lead to severe diarrhea, dehydration, and potentially life-threatening complications if not treated promptly.
Addisonian Crisis: An acute adrenal insufficiency crisis can present with diarrhea, fatigue, dizziness, and muscle weakness. It's a medical emergency requiring immediate treatment.

Porphyria: A group of disorders that can cause abdominal pain, diarrhea, muscle weakness, and neurological symptoms including dizziness.
Mitochondrial Myopathies: Rare genetic disorders affecting muscle mitochondria, which can lead to muscle weakness, fatigue, and gastrointestinal symptoms including diarrhea.
Whipple's Disease: A rare, systemic bacterial infection that can cause diarrhea, weight loss, fatigue, and muscle weakness, among other symptoms.