I have had positive ana for 5 years, getting higher and higher to 1:640. now suddently negative without treatment. how is this possible?

See details: An ANA is nonspecific and can even be seen in normals. It can become negative with time. I would stop testing for it.

Non-specific ANA: ANA can be elevated for a variety of conditions including viral or bacterial etiologies. It does not mean you have an autoimmune disorder. It is a non-specific test. Despite high titers, and now suddenly a negative result without treatment further emphasizes this fact. I would not worry. Treat the patient, not the lab result, especially if you are asymptomatic.