My Mom Had Very Severeuti With Hydronephrosis ... Her Wbc

My mom had very severeuti with hydronephrosis ... Her wbc count went to 34000 and pus cells in urine were 100...She then did meropenem course from last 3 days... Her wbc count come down to 14000 and pus cell counts in urine came to only 2. But esr is still 100 with some fever.. Antibiotics seems to have worked a lot as huge progress was seen after meropenem course... What is reason of her fever and esr now?