Autoimmune diseases are quite common and for some reason doctors are often reluctant to test for them. We've had several instances where they did not test for autoimmune diseases only to turn out that there was an AI issue. Several of the autoimmune diseases can cause pregnancy loss so I think it's extremely important for docs to test for autoimmune disease in any cases of recurrent pregnancy loss.
Autoimmune Disorders & Miscarriage | Prevent Miscarriage
Most autoimmune diseases are silent for many many years before becoming symptomatic. In fact it's very often the case, particularly in younger women that their disease is discovered when they lose a pregnancy or have a child with a health issue. I have an autoimmune disease and my blood tests indicated I'd had it for quite a long time, certainly during my last 3 pregnancies. Just luckily for me that it didn't cause any harm to my kiddos.
This is not a cause for additional concern. Please do not now have more concern than you already have. This is a good test to have, it will help rule out a lot of potentially harmful things (and even if you do have one or more than one as sadly they go in packs, they can treat it) . I'm simply glad that they have tested you for the AI diseases as not everyone gets that test ordered for them.