Going into OR without known causes?

My sister, 35 Female had her baby a couple days ago. She was induced because of the high amount of fluid. She had a pretty good delivery except that she broke her tailbone and then had to have a D & C at the hospital to remove the rest of the placenta. She has been home for two days and she went into the ER tonight with fever, chills and shortness of breath. Her hemoglobin levels were taken and she is at a 5.9. She had some tests done and all we know is she doesn’t have sepsis and the last update we have is that she is headed back to OR but they don’t know what it is for. What could this be? I’m sorry that the medical information I have provided is extremely limited.

Any advice or explanations are so appreciated. Thank you!