FOAMed

A 70-year-old female presents with left lower quadrant abdominal pain for 2 days, gradually worsening until it was no longer controlled with over-the-counter meds. She also mentions that she has not had a bowel movement for two weeks but has been passing flatus. She has not taken her temperature but has felt feverish and chilly; she also vomited twice today (non-bilious and non-bloody). She has no appetite and has only managed sips of water in the past few days. On exam, she appears ill and visibly uncomfortable. She has tachycardia and mild tachypnea. Abdominal exam reveals diffuse tenderness, with maximal tenderness in the left lower quadrant as well as rebound and involuntary guarding. What is the diagnosis?