My MC recently had a fairly major surgery while in an artificially immunosuppressed state. The thing that was causing the immunosuppressive effect (effectively a parasite) left her system immediately after, and her immune system returned to normal levels shortly. She then develops a fever -- I'm waffling whether this is due to something like cytokine release issues from the surgery situation, a drug reaction or an unrelated illness. (I'm leaning toward the first, but since parasites dramatically affecting the immune system and then being removed isn't something that actually happens, I don't know what time frame I'm looking at for cytokine storm issues, what kind of effect on the immune system this fully fictional parasite would have, etc. I am totally, totally willing to scrap this if it's nonsensical.)
However the fever (and other symptoms) are not due to peritonitis, a UTI, or surgical site infection. (I know that an infection is definitely a possibility, but she doesn't have optimal resources regarding treatment, and she's slated to at least survive this a little while.) How soon would it be possible to tell that this was not peritonitis? It seems like the main and obvious indicator is abdominal pain, and checking for rebound tenderness can distinguish between that and abdominal pain that doesn't involve the peritoneum. How quickly does that appear from the first onset of fever/nausea/vomiting, or do they appear together?
Terms Googled: "post surgery fever" and "postoperative fever", which gave me some good resources for how someone with medical training would understand/diagnose/treat postsurgical issues, "peritonitis", "obstetric peritonitis", "peritonitis c section"/"caesarean section", "cytokine release surgical trauma", "cytokine release syndrome", several search terms related to parasites and the immune system.