The difference is if it always hits before your period (may be a little more diagnostically complex if hypomanic symptoms are present though because it's generally depression/anxiety/irritability). Otherwise, I believe it's considered exacerbation of your per-existing condition . However, if antidepressants don't work (or your bipolar and can't take them), that doesn't mean that it isn't PMDD. Some people do not respond to Sarafem/Zoloft. I think they try a specific type of birth control, and there is another treatment which I can't think of off the top of my head. Plus, a bunch of other non-prescription treatments.

My diagnosed PMDD is so very predictable. If I experience any hormonal fluctuations I begin to have rapid mood swings similar to what you are describing. There isn't so much of a euphoria but agitated irritability (depression isn't always present in these states in the traditional sense) to the level I could send tables flying. I feel severe depression, agitation, and I pace, disassociate, sleep less, feel rejection sensitive paranoia, anxiety, etc. These comes in cycles, and in between I am happy and chatty. If they do not resolve, I know that I've just hit a really bad case of it. My functioning hits bottom, and I just wait for it to go away.

Oh, and I'm experiencing it now! How exciting!