A 22-year-old woman presented to hospital 10 days after emergency caesarean section with severe back pain, fever tachycardia and a raised C-reactive protein. She had received labour epidural analgesia and was investigated for an epidural abscess. After repeat magnetic resonance imaging she was ultimately diagnosed with septic sacroiliitis. Although an uncommon cause of back pain, pregnancy-associated sacroiliitis should be considered in the differential diagnosis of post-epidural back pain, as the presentation and symptoms of an epidural infection and sacroiliitis are similar. We recommend imaging to include the sacroiliac joints when considering the diagnosis of an epidural collection.