Should I be worried about this?

I am on an 18-day cycle. I had one period on the 18th of August, then on the 5th of September, and I'm guessing it will be again on the 23... that's TWO in one month. It is very VERY painful, and sucks every ounce of energy out of me. Could it be stress-induced? There is a lot going on at the moment. What causes double month occurrences, and how do I treat it without going on the pill? Due to hereditary female problems ociated with birth control, I refuse to touch that stuff. If there another alternative? Should I wait this out and see what happens in the next few months? Or should I be concerned? Thank you.