What Causes Radiating Pain From The Back Of The Leg Up To Foot?

Hi, This is a suprise, & I wonder if you can give any advice. I am in very good shape, look great, & 49 years old and have recently started to have cramp like pain that runs right down the inner/back of my lower leg from the back of my leg, not as sharp as cramp but very painfull, seems to be triggered off walking down stairs, after having worked on the computer for a while, also appears in the morning after waking, can walk through it, tends to wear off after 5-10 minutes

Thanks

sam