Hello My Son Is 2 Years Old And Was Detected

Hello
My son is 2 years old and was detected with moderate hydronephrosis bilaterally. Kidneys revealed loss of cortico medullay differentiation and increased parenchymal reflectivity. He was detected grade V vesicurteral reflux bilaterally. We have had his reflux fixed with a surgery abt 20 days ago. Since then he had been experiencing pains while urinating. An USD KUB revealed proximal ureter dilated bilaterally, Please csn you advise what must be the cause of paisn while urinating for him still persisiting sonce it is learne that it should go away soon after the surgery.