Funny, i have just posted on anther thread, but you probably won't see this as it is so out of date.
I have exactly the same problem; I have always been a little weaker prior to my eriod, and think I am premenopausal which is why it is starting to get a little weaker in general. It is not stress, Definately hormones, and I would love to know if anyone has any tried and tested sound advice.
The GP said to not avoid drinking as it makes the bladder more irritated.