What would trigger low grade fevers several times a month. Sometimes it passes in a few days sometimes they linger for weeks (all around 100.3) All blood test/thyroid have shown no issues? Just tired

Doctor. YOu need to see what is causing this. This is more about what is not causing this. You need to have a workup of what is the particular disease process or infection that is hard to find. This may be a very difficult thing to find. Despite what you have had you need to be reevaluated again.