A low platelet level can be caused by a number of things including medications, illness or disease. One relatively common condition is called idiopathic thrombocytopenia purpura (ITP), which I suspect you may have. Idiopathic means that something happened for no identifiable reason, so those with ITP have low platelets without known cause.
Most people with thrombocytopenia have enough platelets to get the job done despite the low level, and thus have no symptoms related to the problem. It is not until the level gets to below one third of the bottom of the normal range that blood clotting becomes compromised. At this point, a person may notice bruising with little or no impact, nosebleeds, bleeding gums, etc.
Fatigue is not a known symptom of thrombocytopenia, so this symptom is probably not directly related. There could be a "unifying" cause, i.e. one problem that causes both low platelets and fatigue, so ask your doctor about that possibility.