I don't know anything about PMDD or even much about rapid cycling, but I just wanted to say hi and that I'm sorry you are going through this.
Having PMS is bad enough (if it's related to that?), but also having mood cycles every 4-6 weeks? That's awful.
I'm currently in a manic episode (lasting 6 weeks, well, it's mostly not dangerous so probably high hypomanic), but I know what it's like to be depressed. Most of the past 2 years before this were that way. I'm glad I have some meds now. If only I could come down a little to do the things I love again and stop all these compulsions.
I hope you feel better soon. Mood swings are usually painful and exhausting for me, so I can only imagine.