Positive ana but no autoimmune disease. what could be the reason?

False positive!: Ana's themselves may suggest an immune problem but are almost never diagnostic. For any. Ana's are present in nearly 5 in 100 people with no immune disease! false positives are more common than one with significance!

Then why was it done: What problems or symptoms led to the test being ordered in the first place? That may be key to determining if the ANA has significance or not.