My rule is that anytime a headache lasts for more than a few days or interupts your life it's time to go to the doctor. I have found that both caffeine and the chemicals in diet pop trigger headaches for me.
Do you experience more stress during the month of the headache? Are there any other cycles going on in your body? or in the weather? I find that rain and low clouds start my headaches.
Thank you both for your replies. I'd never thought about diet drinks causing headaches. It's possible that they could be a trigger. My headaches started about 7 years ago, but I've only been drinking diet drinks for about 4 years. However, they could have added to the problem.
I know that sometimes pressure changes in the atmosphere trigger headaches. Also, if I don't eat on a regular schedule, I get a headache. If I sleep later than usual, I get a headache that lasts most of the day.
Thanks for the advice. Hopefully, I will get in to see a doctor soon!