What Causes Irregular Periods After Stopping Demulen?

I have been on the BCP for 17 years. In August, I switched to Demulen but started having bad headaches to was told to stop taking BCP all together. Tests for headaches were done (CT scan, etc) and nothing was found so I am back on BCP, but now Cyclen. Since being off Demulen - about 3 months, I've had my period for approximately 2 of those months. Not heavy. I don't know what to do. Doc wants me to stick with Cyclen for another month to see if it works. Any suggestions?