Causes of blackouts. no alcohol or drugs involved?

Cardiac arrythymia ( like atrial fibrilation or pre-mature ventricular contractions) can cause temporary alterations in consciousness. See a cardiologist for an exam and EKG, as there are other causative factors.

Majority are caused in the neurocardiogenic context and a thorough exam along with detailed history would be the starting point. Your main doc will also have access to your history, meds and family history that will all help in putting together a good plan. The sooner you get examined, the better.

In your age group this needs immediate attention as cardiac and cerebrovascular problems can be underlying culprits and so could metabolic and hematologic issues. See your doctor