Why did I pee instead of nut?

Not sure if 'sex' is the right tag but its the closest to it. Also, might be a tough read

I recently learnt that prenut isn't the end. Last night was my first time going all the way. But once I thought I was ejaculating, it was just piss. A cannon of piss. I didn't know what to do. I just waited for it to be over then washed my sheets and took a shower. Is this normal for your first ejaculation? Was I supposed to pee before starting? Is it some medical thing? I didn't want to search it on google because last time I tried to get a diagnosis I thought my appendix burst. It didn't.