Thanks for all your replies, mamas! It looks like I am not the only one suffering from this. I have done a bit of research on the web and found out it is called PMDD, like PMS only much worse. (Don't let anyone tell you 'It's *just* PMS.) According to various websites, there are quite a few things you can do for it:
1. Meds - specifically Prozac and Zoloft seem to be mentioned a lot
2. Herbal - Evening Primrose Oil, B6, calcium and magnesium (although most websites state that in studies, these don't seem to do much - I guess it is a matter of trying it for yourself)
5. Enough sleep (good luck if you are a mama of a little one - mine is 2 1/2 and has still never slept through the night!! But I do find when I have had more sleep, I feel better. The worst days have to be when I get very little sleep.)
Personally, I take calcium and magnesium (which does help me sleep better) and am trying EPO. I should exercize more, but I live in the UK and winters are a bit grim, so it is hard to find the motivation to go out in the rain. I am still BF'ing but am considering going to my doc for meds that are compatible with BF'ing, as when I get this, it is really, really, really bad. It turns me into a very bad mother.
Fortunately, I am in my good 2 weeks at the moment and enjoying every minute of it. Unfortunately, I know that the dark, bad days are coming, and I will have to get through them before I can have good days again.
If anyone wants to do more research on this, I would suggest searching under PMDD on the internet.
Good luck mamas!!