Oh my I feel your pain!!! I was on fluoxetine (also generic Prozac) for about 3 months and gained 10 pounds as well, AND felt completely lethargic with no energy all the time.
I just recently switched to Wellbutrin (and by recently, I mean 3 days ago) but already I feel like I have a bit more energy. Could be the placebo affect, but I'll take it! I chose Wellbutrin because it has a lower incidence of weight gain and sexual side effects, but I don't know anything about its treatment for PMDD. We'll see how it does with the weight issue.
Good luck to you!! I think you should definitely talk to your doc about other options. PMDD sucks, and so does gaining weight! There has to be a better answer for you.