It's called My Pain Diary and it's so worth having. You can e-mail reports to your doctor if need be. I've been using mine to keep track of all of my pain as I was just recently diagnosed. I'll be printing this off for my doctors to take a look at. I think it's great. You can set yourself reminders as well, it documents the weather (pressure, temperature, humidity, etc.)
If you have an iPhone, I certainly recommend downloading it. It might be on the Android market, too.