Im always moving my left leg. whether im sitting down, laying down or standing up.. but not constantly, people notice i always do this too.

Maybe "nerves": A common cause of constantly moving one limb is nervous tension which has developed into a habit. However, it is important to notice whether you are capable of stopping it, temporarily, if you make an effort. If you can't control it, then it may be a tremor. There are many causes for a tremor, and it would be a good idea to consult a doctor. But if you can stop it, you can break the habit.