Gall bladder Fossa FEVER, ERCP, lap chol

My mother underwent ERCP procedure for CBD stone removal and gall bladder removal surgery next day. Post surgery she has had persistent Fever. Fluid collection was diagnosed a week later from surgery. That has shrunk from 28 cc to 12 cc over a course of Ceftum 500 n farinom 200 taken for 3 weeks . However her fever persisted. Next, Zostum 1.5 gm injection IV was prescribed for 5 days. Yet a day later of injection course ending, her Fever relapsed. That generally hovers around 100°. Her platelet count has varied but has been very elevated post the surgery . TLC last value was 12500.  All her vitals and blood reports were normal a month prior to admission for procedure and surgery.  WHAT COULD BE THE REASON FOR HER  PERSISTENT FEVER ?