Almost two pounds lean gain in one week?

[SunsetRunner]

I have been using a Fitbit Surge for about a month and a half, and I got a road bike about two months ago. I've been constantly losing weight, which is great; however this week something different happen. 

I didn't lose weight this week, but I gained almost two pounds of "lean" which I don't know if this is possible. I pushed myself pretty hard this week, going way over my activity goals for the week and I also added whey to my diet for faster recovery. Even though I never go over my calorie intake which is supposed to be under 1000.

Does this mean I lost two pounds of fat but gained them as muscle? Is this even possible? Did I measure my calories wrong?

Here's some screenshots:

BeforeAfter

Thanks!

Edit: I recently bought a Fitbit Aria to measure my weight which is why I only have a week of data. Should I be measuring every day?

[Dominique]

Don’t take changes in lean mass vs. fat mass as estimated (stress on estimated, because it’s not a measurement) by the Aria too literally. The method used (bioelectrical impedance analysis) has known limitations. Gaining muscle mass is a very slow process, even more so when you are eating at a deficit in order to lose weight. Any larger fluctuation during a short period of time (e.g. one week) is likely to be due to variations in your hydration level and/or to the limitations of the method used.

Since the Aria makes logging your weight and BF% so easy, I personally like to use it everyday. I recommend linking your Fitbit account to Trendweight, a free service that displays your weight and BF data in a more legible way than Fitbit does.

[A_Lurker]

Exactly the same kind of answer that @Dominique made.  My weight flucutates quite a bit but bodyfat and lean mass can be all over the map.  Hydration plays a big part of that.  Using Trendweight though I can see overall trends.  I'll be happy maintaining as much of it as I can as women have a harder time building it, so not losing it is important to me.  I was lifting fairly heavy for a bit, but not doing anything specific at the moment (and won't likely until the new year).

If you look at my trend (the dark line) is surrounded by lots of actual readings.