Don't panic! Weight changes day by day, in fact probably hour by hour. Sometimes you might be retaining water a bit, or something else... I mean we've all had times when we felt bloated etc. The main thing is, you're sticking to the plan and that means you'll lose weight overall.
Daily trips to the scales are a bad idea btw, they'll only freak you out. Try to wait until WI day to find out how you are doing. And always get weighed in, because that's a chance to review your diet/progress with your consultant.
Gaining truly sucks but not going to SW because you are having a bad week will only result in you giving up. Plus, if you stick at it and face any setbacks, you'll be an inspiration to other people in your class, who might face the same issues in the future.