Getting an erection when I need to pee, anyone else get this?

Like I was driving on a long car journey the other day, and I needed to take a piss. I waited so long before I stopped at a rest area and went that I had an erection. I've got an erection before when I didn't use the restroom for a while then too. Does this happen to anyone else, and is it normal to get hard when you haven't pissed in a while?