Ectopic Pregnancy: Given the symptoms of left-sided pelvic pain, ovulation from the left ovary, and the presence of endometrial thickening without a well-developed corpus luteum, an ectopic pregnancy is a strong consideration. The mild left fallopian tube thickening further supports this possibility, especially in the context of a negative pregnancy test and missed period.

Corpus Luteum Cyst: The presence of endometrial thickening and the fact that the corpus luteum is not well-developed could suggest a corpus luteum cyst, which can cause pelvic pain and irregular menstrual bleeding.
Ovarian Cyst: A cyst on the left ovary could cause pain and might not be fully visualized or recognized on the scan, especially if it's small or not significantly impacting the ovary's structure.
Pelvic Inflammatory Disease (PID): Although less likely given the normal scan findings, PID could cause pelvic pain and mild fallopian tube thickening. However, it typically presents with other symptoms such as fever and abnormal vaginal discharge.

Ruptured Ectopic Pregnancy: This is a medical emergency that requires immediate attention. Even though the scan shows mild findings, a ruptured ectopic pregnancy can present with sudden severe pain and could be life-threatening if not promptly diagnosed and treated.
Ovarian Torsion: Although the scan does not show significant ovarian enlargement, ovarian torsion can occur without obvious signs on imaging and is a surgical emergency.

Mittelschmerz: This refers to ovulatory pain and could be a consideration given the timing of the pain relative to ovulation. However, it typically does not cause significant endometrial thickening or fallopian tube changes.
Endometriosis: While endometriosis can cause pelvic pain and endometrial thickening, it is less directly linked to the specific timing and location of pain described, and the scan findings do not strongly support this diagnosis as the primary cause of the current symptoms.