You were the new addition of the 501st. You weren't a clone or Jedi though, you were a separatist droid. You were reprogrammed to serve the Republic, and to not create any confusion, Anakin Skywalker introduced you to the 501st

Anakin:"Here it is- our new companion."

Rex:"Uh, sir-...do you remember what happened as we trusted R3?"

Fives:"You can't be serious, General..."

Kix:"I'm no droid medic, so if this one goes down, I won't take care of it."

Tup:"I trust the General with this one..."