Ovarian Cyst: Given the presence of deep tenderness and the negative beta hCG, an ovarian cyst is a plausible cause. Ovarian cysts can cause pelvic pain, especially if they rupture or torsion occurs.

Constipation: Although not directly related to reproductive issues, constipation can cause significant pelvic pain and deep tenderness, especially if there is a large fecal impaction.
Urinary Tract Infection (UTI): A UTI can cause pelvic pain, and the absence of menstruation does not rule out this diagnosis. The pain could be referred from the urinary tract to the pelvic area.
Diverticulitis: If the patient is of an appropriate age, diverticulitis could be a consideration, especially if the pain is more localized to the left lower quadrant.

Ectopic Pregnancy: Although the beta hCG is negative, very early ectopic pregnancies might not yet have produced enough hCG to be detectable. This diagnosis is critical to consider due to its potential for severe consequences if missed.
Appendicitis: Appendicitis can present with pelvic pain, especially if the appendix is located in a retrocecal position. It's a surgical emergency if not promptly addressed.
Ovarian Torsion: This is another emergency that requires immediate attention. The twisting of an ovary can cut off its blood supply, leading to tissue death.

Endometriosis: While endometriosis is not rare, it's less likely to present acutely. However, it could be considered in the differential, especially if the patient has a history suggestive of endometriosis.
Pelvic Inflammatory Disease (PID): PID could cause pelvic pain but typically presents with other symptoms such as fever, abnormal vaginal discharge, and multiple sexual partners or a new partner.
Malignancy: Although rare, pelvic malignancies (e.g., ovarian cancer) could cause chronic pelvic pain. This would be less likely in the absence of other symptoms like significant weight loss or a palpable mass.