Gastroenteritis: This is a common condition characterized by inflammation of the gastrointestinal tract, often due to viral or bacterial infections. The symptoms of nausea, vomiting, and abdominal pain are classic for gastroenteritis, and the absence of fever does not rule out this diagnosis, especially if the cause is viral.

Food Poisoning: Similar to gastroenteritis, food poisoning can cause nausea, vomiting, and abdominal pain. It is often related to the consumption of contaminated food and can be caused by bacteria, viruses, or other toxins.
Irritable Bowel Syndrome (IBS): Although IBS is a chronic condition, an acute exacerbation could present with these symptoms. However, the duration and absence of other IBS symptoms (like altered bowel habits) might make this less likely.
Appendicitis: Early appendicitis can present with nausea, vomiting, and abdominal pain, often starting around the navel and then moving to the lower right abdomen. The absence of fever does not exclude this diagnosis, and it's crucial to consider appendicitis due to its potential for serious complications if not treated promptly.

Appendicitis: As mentioned, appendicitis is critical to consider due to its potential for severe complications, including perforation and peritonitis, if not promptly treated.
Intestinal Obstruction: This condition, where there is a blockage in the intestines, can cause severe abdominal pain, nausea, and vomiting. It is a medical emergency and requires immediate attention.
Ectopic Pregnancy: In women, an ectopic pregnancy can present with abdominal pain and nausea. Although less common, it is a life-threatening condition that must be considered, especially if the patient is sexually active and has a missed period.
Pancreatitis: Acute pancreatitis can cause severe abdominal pain, nausea, and vomiting. It is a serious condition that requires prompt medical attention.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can present with abdominal pain, nausea, and vomiting, but they are less common and typically have a more chronic course with additional symptoms like diarrhea and weight loss.
Mesenteric Lymphadenitis: This condition involves inflammation of the lymph nodes in the mesentery (the tissue that attaches the intestines to the back of the abdominal wall) and can mimic appendicitis. It's more common in children but can occur in adults.
Gallbladder Disease: Gallstones or cholecystitis can cause abdominal pain and nausea, but these conditions are less likely in a young adult without other risk factors.