Yep- I hear ya! I used to work in a place where they had a staff birthday party once per month which were mandatory to attend and of they course served huge cakes. There were always doughnuts in the kitchen and chocolates/candy dishes at everyone's desks.
It is so hard to say no, then if you do say no to birthday cake every asks why not?
My strategy was to tell them I was full from lunch, or that I could only have a teeny slice since I get headaches if I eat too much sugar - that one got me out of the doughnuts and candies too
I find it helps to think in a positive term too - instead of saying "I can't have that!" I tell my self "I can have that if I wan't but I don't really need it".
It is quite normal to have weight gain during PMS - you get bloated and it shows on the scale, but then the following week it will be gone.
If you need chocolate for your PMS then try a diet hot chocolate or a fat free chocolate pudding. I usually crave crunchy salty stuff so I have some lower salt triscuits with low fat cheese to get by - usually does the trick!