Cheesy though it may sound, there's a good chance it's all in your head. It sounds like what I get from time to time (I have mild body dysmorphia, like the majority of women in the Western world). Even a slight change in skin-tone can make a woman feel thinner or fatter. You'll probably find that, if you take exact measurements every morning, fat levels go up and down over a week or so, but that the daily changes you're perceiving aren't actually happening or as extreme as you feel.
If it's real, it is probably just water retention, mind. Either way, not much to worry about.