I have epilepsy and have had it ever since i was 12 years old. i'm almost 18 and haven't had a seizure in three years.. except for yesterday. why now?

Unpredictable: It is hard to say without knowing more about your seizure history and current medical status. Important to see your primary care physician and/or neurologist for more clarity.

Start medication: Seizures are sporadic and can be triggered by infection, stress, fatigue, etc, and not always clear as to triggering event. However, you are best protected if you restart an anti-epileptic drug. Maybe, you will stabilize, and perhaps can stop meds after age 21 or 22, but work closely with a neurologist.