You haven't done anything wrong, it's just the way it goes sometimes. It could be down to a myriad of things, too much salt the day before, water retention, TOTM etc. Over the next few weeks and months you will probably start to see patterns, then sts, small losses and gains become easier to accept as you know what's caused it.
And, don't be so hard on yourself. If someone had said to you three weeks ago ' follow this diet and you will be 8lb lighter' you would have said 'wow that's great'. If that same person said to you 'ah but you will lose 3.5, 4 and then 0.5'. I'm guessing your reply would have been 'Thats ok 8lb is great overall'. And it is! Just keep up what you are doing and the weight will sort itself out.
As long as I am lower at the end of the month than the beginning then I am happy. Because all I expect and hope for is a downward trend.
Good luck for next weeks weigh in