Mine are associated with hormones, air pressure, and certain types of stress. I used to have some clue when I'd be in the 'zone' because of predictable hormonal fluctuation. Now, I'm of an age when my hormones are going batshit, so the migraines just surprise me.

Air pressure: storms kick of my migraines. Sometimes before, sometimes during, and sometimes after. I think it's the changes in air pressure.

Stress can do it, but not all types. I hardly ever got a headache from work: racing to meet a deadline, butting heads with people, or any sort of 'worry'. Driving long distances and lack of sleep, on the other hand, have a high probability of causeing a migraine.

Diet is usually considered to be a big factor, and there are certain known 'trigger' foods such as red wine, tomatoes, and chocolate. I haven't found that food make much of a difference for me but it does for many migraineurs. Caffeine withdrawal can cause headaches, and caffeine can help make them go away.

One other thing - there's such a thing as 'rebound' headaches that feel a lot like migraines. They're actually caused by various drugs used to treat headaches (anti-inflammatories, and analgesics) and can get you stuck in a cycle.

If this isn't just a one-off episode, I'd check with my doctor. There are loads of prescription meds that can help prevent migraines, and one herbal preventative - feverfew. It works for me and I've never had any side effects. It's also recognized by main-steam doctors (at least in the US) as being effective.