I haven't been physical active in months. now i'm very fatigue. have weakness in legs lack energy & strength. can this be from being a couch potato?

Possibly: Certainly, lack of activity can be a cause but if your symptoms are really significant then you need a medical evaluation to rule out other causes.

Yes: Significant peroids of inactivity allow the muscles to loose strength and stamina in a short period of time in some cases. Prolonged inactivity has negative affects on respiratory and biochemical functions of the body.