This happened to me last week. I was good all week, then I gained 3lb. The week before I had been an angel too so it can't have been the after effects of a bad week. I stepped off the scales so annoyed and when my consultant was talking to me in image therapy, I had to fight back the tears. She asked me all sorts of questions to try and understand why I had gained but there was no reason. I drink plenty, I measure and weigh my healthy extras and my syns. I am constantly checking things on lifeline online. I eat my super free. If it had been half a pound I would still have been annoyed but I wouldn't have been so upset.
It wouldn't be so bad but I have been following the plan for 10 weeks and I don't seem to be getting anywhere. I have (after the gain) only lost 2.5lb. I know it works as I followed the plan several years ago so I'm not sure why it's not working at the minute.
I haven't let it affect this week tho. I have continued to follow plan. I am still weighing and measuring healthy extras and syns and checking everything online. If I don't see a loss this week however, I'm not sure I will be so strong.