there are a lot of recreational substances that can stop you up in that area. I did a lot of opiates for a lot of years and you can spend hours trying to empty your bladder.
what I discovered is to dangle the tip of your johnson in some warm water (note - water from the tap can be hot enough to burn you down there so be careful), closer to body temperature the better. relaxes things right up
You are correct it is bladder not the kidneys. It is called retrograde ejaculation.
Either way it wasn’t something I wanted to happen so I took a few weeks off wanking while I was in pain from the kidney stones.