I think Sveta's answer supports what others have been saying. Sveta is tall and statuesque. A 5 lb weight fluctuation on her is barely going to register. I, on the other hand, am 5'1". A 5 lb weight fluctuation on me = too-tight pants. The shorter you are, the more likely it is that a minor weight change is going to affect your pants size.
Having said that, my weight remains remarkably stable. I have all different sizes of jeans, ranging from 24 to 27, but that's just manufacturer's differences. All my jeans always fit me. BUT, I have (according to my doctor) "hormone-sensitive breasts", which means they swell up a lot for the second half of every cycle -- enough that I wear a bra with slightly larger cups during those two weeks! It's quite funny, actually. Just because the same thing doesn't happen to all of my friends doesn't mean I'm not normal. It's just a peculiarity of my body. My waist and hips do not seem sensitive to monthly hormone fluctuations, although I know that's a more common issue for other women. So there ya go. Keep a couple of different pants sizes, and don't worry about it.
