Viral Gastroenteritis: This is the most likely diagnosis given the acute onset of severe diarrhea, headache, and fever, which are common symptoms of viral gastroenteritis. The fact that the patient can only tolerate bland foods like bread and crackers also supports this diagnosis, as viral gastroenteritis often leads to a temporary reduction in the ability to tolerate a wide range of foods.

Bacterial Gastroenteritis: Similar to viral gastroenteritis but caused by bacteria. Symptoms can be more severe and may include blood in the stool. The high frequency of diarrhea (10-15 times per day) could suggest a bacterial cause.
Food Poisoning: Although the patient mentions she can only eat/tolerate bread and crackers, if she recently consumed contaminated food before the onset of symptoms, food poisoning could be a consideration.
Inflammatory Bowel Disease (IBD) Exacerbation: If the patient has a history of IBD (Crohn's disease or ulcerative colitis), an exacerbation could present with severe diarrhea, although other symptoms like blood in the stool and weight loss might also be expected.

Clostridioides difficile (C. diff) Infection: Although the patient has not used any antibiotics recently (a common predisposing factor for C. diff), this diagnosis is crucial not to miss due to its potential severity and the need for specific treatment.
Cholera: While less common in many parts of the world, cholera can cause extremely severe diarrhea and is life-threatening if not treated promptly.
Intestinal Parasites: Certain parasites can cause prolonged and severe diarrhea. Travel history or exposure to contaminated water would increase the likelihood of this diagnosis.

Ischemic Bowel Disease: Although rare, especially in younger patients, ischemic bowel disease can cause severe diarrhea among other symptoms, due to reduced blood flow to the intestines.
Hormone-secreting Tumors (e.g., VIPoma): Very rare tumors that can cause severe, chronic diarrhea due to the secretion of vasoactive intestinal peptide (VIP) or other hormones.
Autoimmune Disorders: Certain autoimmune disorders can cause gastrointestinal symptoms, including severe diarrhea, though these would typically be accompanied by other systemic symptoms.