I don't know either, but I do know that I sometimes had trouble peeing when pg, and sometimes I even find it a bit at the start of my period. I put it down to the uterus blocking the the urethra somehow but I am only guessing. I did read that leaning forward a lot can help when weeing during pg if you have an inverted uterus. I didn't have an inverted uterus, but I did find that helped a little bit.

It is possible that it was just the position of the baby last time and it might not happen again.