I have something called exertional compartment syndrome (I know it sounds made up!) It is worse when I weight more, and better when my weight is down. It causes numbness in my feet as well as severe pain in my shins and on the sides of my legs. They get really, really hard too. Basically what happens is all of the blood is pumping down to my legs, and my fasica isn't flexible enough and it ends up constricting the veins so the blood doesn't flow back up my legs (it just all kind of stays down there until I stop the activity). Usually resting for a bit, walking it off slowly helps. At first I went to the local shoe store, and got fitted for a new pair of shoes, thinking shin splints were the problem, but that wasn't the case. It could be that there is just a lot a pressure in your legs and it is causing the numbness or discomfort.