Interesting, it took me a bit to figure out what was wrong, so I can definitely see why you're struggling with it!
The length proportions are all fine in your drawing. I think it's the shape of the lower leg that makes it look off.
In your drawing, the lower leg is very round in shape and stays relatively thick from the calve up to the ankle. Take a look at this (watermarked) stock photo:
http://i.istockimg.com/file_thumbview_approve/9540560/2/stock-photo-9540560-classic-female-leg-posing.jpg
Notice how the leg considerably slims down at an angle halfway down the lower leg? In your drawing this doesn't happen. It starts tapering down at the ankle, when it should start tapering halfway down already. I think if you fix this, your drawing will look a lot more anatomically correct!
There are two more small things that I noticed:
1. I'm unable to tell if the line you drew down the center of her lower leg is a sketch line to guide you, or it supposed to outline the bone/muscle in the leg. If it's the latter, try making it straighter. It doesn't curve with the fat deposit and muscles, so if you straighten that out it should make it look a little less awkward. If it's a sketch line, then it's not directly relevant to your drawing!
2. Feet are roughly the size of the lower arm, starting at the inside of the elbow to the wrist. (I use this as a measuring tool when I go shoe-shopping all the time!) Unless she has unusually small feet, try making her feet a little longer to fit with this proportion so she looks more balanced. :)
All in all I really enjoy the drawing, you seem to have a good grasp on pretty much everything!
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