It's a nice capture for a "Hey, look at me" moment. I actually like the tree as a background. You got some fortuitous lighting at her chest level, so the black of her vest doesn't fade into the tree, and there seems to be a bit of rim lighting separating her hair from the tree. Your DOF is nice - all of her seems sharp while the relatively nearby tree is sufficiently out of focus that she pops. What I don't like about the background is that it's split almost equally between tree and grassy area, and I agree with Mike that having her straddle the transition so that it comes out of her head is awkward (such is the price you pay for spontaneity, and I expect had this been a planned pose you would have composed differently). I think the "grassy area" side is a bit busy (I think I'm seeing a walking path, a stand of trees, and a building in just that little portion), which is part of why I lean towards having more tree rather than less in the background.
In my decidedly non-expert opinion I think you did a damn fine job with the fill flash. If this were a planned shot I would suggest using a reflector to bounce some natural light onto her front. From the light/shadow under her arm it looks like the sun is nice and low, casting a rich light onto the scene - it would be nice to get some of that quality in the light on her face.
You got lucky with the crop of her left arm - you just missed cutting her off right at the elbow. I like the horizontal orientation, but perhaps starting from a slightly wider view would have let you get just a little more into the bottom of the frame to get just a bit more of that forearm.
