See your doctor, or an ob/gyn.
It's one thing to be irregular. It's another to not have had a period in about 6 months. I'm not saying you're pregnant, but you DO need to find out why you aren't having your period.
Expect some blood to be drawn for labs. I'd be curious to see if you were anemic.
What is your blood pressure?
Again, you need to see a doctor - NOW.