Endometriosis: The patient's presentation of pelvic pain, along with the ultrasound findings of a thickened endometrium (0.86cm) and a right ovarian cyst with internal echoes, suggests endometriosis, particularly an endometrioma (a type of ovarian cyst associated with endometriosis).

Ovarian Cyst: The presence of a right ovarian cyst with internal echoes could also suggest a benign ovarian cyst, such as a dermoid cyst or a hemorrhagic cyst, which can cause pelvic pain.
Adenomyosis: Although not directly indicated by the ultrasound, adenomyosis (a condition where tissue similar to the lining inside the uterus grows into the muscular walls of the uterus) could contribute to pelvic pain and a thickened endometrium.
Pelvic Inflammatory Disease (PID): PID could cause pelvic pain, but the absence of adnexal masses or fluid in the cul-de-sac makes this less likely.

Ovarian Torsion: Although the ultrasound does not show any signs suggestive of torsion (like a large cyst or significant free fluid), ovarian torsion is a surgical emergency that can present with sudden onset pelvic pain and must be considered, even if less likely.
Ectopic Pregnancy: Given the patient's age and presentation with pelvic pain, ectopic pregnancy must be ruled out, especially if there's any chance the patient could be pregnant.
Malignancy (Ovarian Cancer): While less common, ovarian cancer can present with pelvic pain and a complex ovarian mass, making it crucial not to miss this diagnosis.

Germ Cell Tumors: These are rare types of ovarian tumors that can present with pelvic pain and a complex ovarian mass.
Stromal Tumors: Including granulosa cell tumors or Sertoli-Leydig cell tumors, these are rare ovarian tumors that can cause pelvic pain and hormonal imbalances.