Macbook Pro 13" retina

I've had this macbook for a good 3 weeks. And it's very nice.

But one thing that's worrying me is the constant fluctuation of my battery life.

I do watch videos and realize I go 4 hours on half-full brightness with facebook being my only other tab.

Then I charge it and I use it while it charges. Sometimes I sleep at 2 and take it out when I wake up at 7.

Other times I sleep around 11 and take it out at 7 or 9.

Now I'm using my mac with lowest, visible brightness, chrome and powerpoint.

I've lost 7% in about 10 minutes.

Is this normal?