What is d' reason for fever, though under antibiotics, after being for detected gallbladder stones?

Before 30 days, the patient had severe abdominal pain and after sonography, multiple gallbllader stones, max. of 6-8 mm., were detected. Since then, 3 to 4 types of antibiotics are given but he has fever,99* to 101*, almost every day, which is controlled by tab. of paracitamol-500mg. Doctor says that some infection is there. Initially, it was told that there is no infection in gallbladder. Then, what could be the reason? Blood Tests and urine tests are done thrice. What should be done?

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