Lurker, but when this happens to me it usually means I have a sinus infection. Sinus infections give me terrible, daily headaches until I get antibiotics to clear it up. Does it hurt more when you bend forward or after you sneeze?
For me, my triggers are too much sleep (sleeping in on the weekends or waking up and then falling back asleep is a guaranteed migraine), alcohol, strong smells (perfumes, incense), and hormones. Another factor that happens less frequently is the drop in pressure that happens in the Fall-Winter transition and Spring-Summer transition. something I've found that works for me is actually drinking a small amount of caffeine each day -- about 6-8 ounces of Diet Coke will stop a headache in its tracks.
When I do get one, I take an Imitrex with Reglan and if I catch it early enough it helps.
It's incredibly frustrating to not know your triggers; it took me forever to figure mine out. I agree with others to start a journal tracking food, drink, lifestyle. It could help connect the dots.
I used to suffer from migraines often. I eliminated all artificial sweeteners and I rarely even have a headache now. Try getting the caffeine fix from something other than diet soda. The only thing that causes issues for me now is flickering lights. I do try to stay hydrated as well. My grandfather needed to be active or he would get migraines. He played tennis until he was in his 80s. :-)