What Causes Crying In A Child Before Passing Urine?

Hello doctor. My six month old son is crying at night before passing urine. This is happening only at night and he stops crying after doing it. I told my pediatrician, but he said that he may not be passing urine for a long time at night, bladder must expand and hence he cries. I have recently started using pull up diapers. I am worried if that is the reason behind his crying. What should i do?