What Causes Migraines?

I'm 30 years old and have been diagnosed with migraines. I have had a terriable headache for the past three days. It is on the left side on my head in the back. If I take a percocet, it helps, but as soon as the pill wears off, the headache is back. It is a horriable pulsating pain and I can't do anything. I can't keep taking heavy drugs to cover it. What do I do?