Hello~I know this was posted over a month ago, how are you now, I hope better. Since the ultra-sound did not show a clot, I would not worry, quite possibly the pain could be due to favoring one leg over the other, thus putting more strain on it. It could also be due to a pinched nerve in your back. If you are still having this issue, I suggest seeing a chiropractor, possibly some adjustments might help clear up the pain. The chiropractor will take an x-ray of your lower back first however, discuss the results and then the treatment plan.
I hope you feel better soon.