Depending on what you are before your appt may effect it as well. When I was pregnant with my first, I remember I had an appt and the lady asked me if I had something sweet for breakfast, and I had eaten a big bowl of fruity pebbles, and she said there was some sugar in my urine, but that was likely why. I only had it that one time and had already had my glucose test. 

Did you have your glucose test yet? I started dropping sugar in my urine around 32 weeks and I had gestational diabetes...

I've had that happen if I had a sugary breakfast. They might have you do another gestational diabetes test of you continue to have sugar in your urine.