Leg numbness in afternoon

[MattMor999]

I am a fit 49 year old. I Swim, run and hike quite abit (4-5 times a week). Anyways, about 5 months ago I started to get leg numbness in the afternoon while walking. It usually comes on after sitting for some bit in the afternoon and then trying to walk. In the morning (when I exercise) there have never been any problems, but if I have been sitting in the afternoon and then try to walk 10 blocks or so, there is often (not all the time) is leg numbness that comes along. The numbness starts in my calves and will go down to my feet (in both legs). As soon as I sit down the pain and numbness goes away. I really do not have any other symptoms.  I have been to the doctor and she did a scan of my legs to make sure I did not have blot clot or something like that. They found no problems. I have never smoked, not overweight, and I am pretty fit (I will swim in the ocean for 2 miles, or hike miles up a mountain with no problem), so I do not think is PAD, but I still seem to have this problem.  Any thoughts as to what this might be?