If you know the reason that you have gained, or at least not lost, then that's OK, but when you don't lose or you actually gain it is frustrating indeed. Seems like Sami dealt with this well. However I just wanted to mention something I learned a few years ago. I had started volunteering to help the leader at WeightWatchers and after a while she showed me how to weigh people, and I did this for a year or so (this was before I was diabetic). So people would come along and have either not lost or gained and said "Oh, but I have been so good!" Inevitably they would have a really good loss the following week. So even though you may be disappointed don't give up because it may be a blip and can often just be down to a small thing like water retention on that day that you weigh.
It was the WW leader who, having noticed my rapid weight loss, told me to see the Dr as she thought I might be diabetic. I was.... BGs were off the scale that could be measured by a meter. I'd been stubbornly ignoring all the very obvious signs.