Why does my leg feel like dead weight?.

My left leg is constantly bothering me.  It kinda feels like its dead weight and i'm constantly aware of my leg.  Sometimes i'll get shooting pain down my leg.  Right now i'm having sensations behind my knee and running up the back of my hamstring.  This pain really hasn't limited me physically.  I practice yoga daily and my leg holds up during classes but i defintely know something is up.  I had this once before a couple of years ago and it lasted for months and months and it just disappeared without me noticing.  I've been told it could be sciatic.....I would appreciate any input.