You're not necessarily doing anything wrong - except expecting the scale to "reward" you when you expect it to (whether it's every day, week or however often) for doing nothing wrong.
Fluctuations happen, and some bodies fluctuate more than others. I'm convinced that if I ate absolutely nothing I would still gain at least 5 lbs during TOM (as long as I was drinking water to retain).
The myth that weight loss should be steadily progressive and routinely rewarding (at least on a weekly weigh-in schedule) is a dangerous one. I think it's one of the leading cause of diet frustration (which I also believe is the leading cause of diet abandonment. It's not the hard work involved - it's the not seeing the expected results of that work. Unfortunately it's not because the results aren't there - we're just no seeing them when and where and to the degree we're expecting them.
Sad News! Weight loss isn't steadily progressive. Not for most of us. It doesn't have to be frustrating, it really doesn't. Frustration comes in when we don't get what we expect.
Imagine that you've only ever gotten paid by the week, and you get a new job which pays monthly or quarterly - only you weren't aware of it. How many weeks are you going to work before you give up (because with weight loss there's no way to ask the boss "when do I get paid?"
If you expect a weekly or a daily pay-off, you're going to be severely disappointed, and as a result severely frustrated, "maybe I am just supossed to be fat forever" is a way of telling yourself "this isn't working the way I want it to, the way it's "supposed to" so why bother."
The problem is we're telling ourselves a very big and terrible lie. It's not "supposed to" be linear, but we want it to be anyway. We're encouraged to want it. We're encouraged to think it's right and normal and if we don't get it, we must be doing something wrong and must work harder to MAKE it work (and it still doesn't).
Change your expectations. Stick to your plan and change your internal "schedule." If you're weighing daily, that's fine but don't compare today to yesterday. Compare today's weight to last months.
Some people are on monthly "paydays." I've even met people on bi-monthly paydays - boy that must suck. To work hard and not see results for six or seven weeks.
If you still aren't losing weight after more than a month on plan, you'll have to make additional changes, but it doesn't mean you were doing anything "wrong" before. Maybe you're not cutting calories enough. Maybe you're cutting them so far that you're exhausted and aren't moving as much (except during your exercise sessions), or aren't sleeping as well, or a billion other things that can affect your metabolism. But you can't declare failure by comparing today's weight to yesterday's or last week's.
Women need at least a monthly cycle, so you have to be on plan at least three months to really even begin to understand your cycle patterns (some people experience one water weight gain week per month, some women experimence two, and some experience none).
Experimenting is the only strategy you've got, but you have to go in willing to look for patterns, not determine ahead of time what they're supposed to be and try to somehow force yourself into the pattern you want (I know it's the way most of us have been taught to diet, but it's also the way most of us have been taught to quit a diet too. Most diets fail, and it's not a stretch to suspect that false expectations are a major cause).