OK ... here we go ... Welcome to 3FC by the way
Would PMS have anything to do with this? Absolutely! From what I've read here on 3FC and what I've experience, ladies gain somewhere in the vicinity of 2-10 lbs sometimes. That is plenty enough to seem discouraging.
How much weight do I subtract when I weigh myself clothed? I'm going to recommend that you try not to ... if that's not possible, try to wear exactly the same thing each time, then it doesn't really matter how much your clothing weighs. A loss is a loss.
Is weight lifting or cardio more important? Neither er both ... While cardio burns calories, muscle strengthening builds a nice muscle base that uses more calories than fat. You can effectively use some of your cardio time, to do a few resistance exercises, several times a week. It's so good for your bones, your shape and your health in general. I like Taurie's suggestion about circuits ... fun, never boring.