First off, I don't want to say I'm sorry because I'm sure you don't want to feel pitied, but I respect you for your bravery to spread awareness of something a lot of people aren't familiar with.
I did a quick Google search and found this website:
Statistics by Country for Fibromyalgia - WrongDiagnosis.com
Unfortunately, it's extrapolated data, meaning the average rate for cases in the US were cross referenced to the Japanese population, giving an average estimate that could be higher or lower. With that data, however, the website gave a rough estimate of just over 1,700,000 people who could potentially have fibromyalgia in Japan.
I could only figure that since that number is fairly small in comparison to the overall population (and that the data is proportionate), you may face about the same climate as you would asking back home.
This is all speculative, of course.