Does being positive ana automatically mean you have lupus or another autoimmune disease? and can it go negative?

ANA is nonspecific: A positive ANA by itself is not useful diagnostically. As with most things in medicine it has to be interpreted in context. Therefore a positive ANA alone does not mean a diagosis of lupus is established. As to your second question, yes it is possible for a positive ANA to later test negative. I suggest talking with your doctor in person about this in your case specifically, and why it was tested.