What Causes Tingling In The Knees Down The Legs?

I am 35 5 3 108lbs, no health problems. I have been experiencing tingling in my legs, from the knees down. This tingling turns into numbness in both feet. No pain associated with numbness. It was first noticed about two months during a triathlon. I thought perhaps the numbing was from the cold weather on race day however the numbness continues. Also might want to note that my weight seems to be dropping despite any effort to loose weight or change my acitivy level. Normal weight for me is about 118.