That is good news, fox. Not good as in 'ah yes, a cure is right around the corner as happens at the 40 minute mark in every episode of House' but good as in you have an avenue to explore and you can predict when things are going to be bad and work around it.
I don't have a lot of knowledge of period related conditions (one thing I haven't looked into for my dh!) but will have a look in my fibro book to see if it talks about supplements and so on that might be useful for period time.
The joint aspect does make me think rheumatology is worth pursuing still, but I think you will need to be referred by your GP, or go for one of those BUPA "GP" sessions and see what they say. How is the fatigue, does that seem to come and go with the menstrual cycle too?