Wow, so this is actually, I think, really difficult to critique, because honestly, you're so technically proficcient, that there are only small nit picks I can point out.

I have one problem, and it may just be me not quite getting your perspective, but in my opinion, the arm which is lifted looks a little bit small to me. Actually, it looks almost in proportion to her head, but too small for the rest of her body. Honestly, that's my only problem, and it took me ages to notice even that!

I love that you didn't make her skinny, or pinup-proportioned, I think her pear shape is lovely, and more original than the more common "big breasts, tiny waist, wide hips, tiny legs".

Her face is very sweet, especially her little nose, and I love the way you draw eyes.

I really like that even though this is a sketch, you made sure to include details on her waistband, the frayed edge of her shorts, and the little knot on her top.

Overall, I think this is original, sweet, and just goes to show how talented you are - it's a simple image, but you've done it beautifully.