A 22 year old lady presents with a 1-day history of right-sided lower abdominal pain and nausea. She denies any urinary symptoms, vaginal bleeding or fever. She is sexually active with a recently normal STI screen. Her menstrual cycle is regular every 4 weeks, but was recently delayed by 2 weeks.
On examination, her abdomen is tender on palpation of the right iliac fossa with some guarding. Observations are all stable.
What is the most likely diagnosis?