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!