How soul destroying for you! This has happened to me in the past. I went to weight watchers and on my first week of dieting I'd gained 2 pounds!!!! I truly had stuck to it! There are several things that can cause problems, such as hormones, fluid retention. In my case it was becuase I was on some anti-depressants and they were causing me to gain weekly. A fact that wasn't revealed until 20 pounds later.
Either way, don't let it get you down. Easier said than done I know! But try and look at the long picture. You are just looking at ONE week. Over time the scales will even themselves out again. Probably next week.