What Causes Dysuria With Swollen Urethra In A Child?

3 yr old boy healthy full term twin has 300 plus protein in urine and dysuria with swollen urethra evening only. Scheduled for urethra repair surgery in 2 weeks. Next week we see nephrologist. Awaiting lab results. Bladder ultrasound clear. Any leads why pain only in the evening? Sleeps through the night and no pain until late in the day. Was fully potty trained but we are back in diapers for urgency when it hits, otherwise I think it just for convenience. Thanks, Concerned mom