Primary Dysmenorrhea: This condition is characterized by crampy, lower abdominal pain that occurs with the onset of menses, often accompanied by nausea, vomiting, and sometimes diarrhea. The patient's symptoms of diffuse abdominal pain worsening at the start of menses and vomiting are classic for primary dysmenorrhea, making it the most likely diagnosis.

Gastroenteritis: Although the patient is afebrile, viral gastroenteritis could still be a consideration, especially if the vomiting is prominent. The abdominal pain in gastroenteritis can be diffuse and may worsen with menstrual cramps.
Constipation: Severe constipation can cause diffuse abdominal pain and vomiting, and hormonal changes during the menstrual cycle can affect bowel habits.
Ovarian Cyst: Rupture or torsion of an ovarian cyst can cause severe, acute abdominal pain that may radiate and could potentially worsen with the onset of menses due to hormonal changes. However, this would typically present with more localized pain and possibly a palpable mass.

Ectopic Pregnancy: Although less likely given the patient's presentation and the fact that she is having her menses, an ectopic pregnancy can present with abdominal pain and vomiting. A pregnancy test should be considered, especially if there's any suspicion of pregnancy or if the patient's pain pattern changes.
Appendicitis: While appendicitis typically presents with localized right lower quadrant pain, early on it can be diffuse. It's crucial not to miss this diagnosis due to its potential for serious complications if not treated promptly.
Intestinal Obstruction: This condition can cause severe abdominal pain and vomiting. Although it might be less common in a young female without a history of abdominal surgery, it's a diagnosis that cannot be missed due to its high morbidity and mortality if not treated.

Endometriosis: While endometriosis can cause cyclical pain, it's typically more chronic and might not fully explain the acute presentation of vomiting. However, in severe cases, it could contribute to the patient's symptoms.
Mitelschmerz: This is ovulatory pain that occurs mid-cycle, not typically at the onset of menses, but could be considered if the patient's menstrual cycle is irregular.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause chronic abdominal pain and vomiting, but they would be less likely to present acutely in relation to the menstrual cycle without prior symptoms.