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but I've been exercising my stomach muscles, etc.
This could possibly be your culprit. Your stomach muscles (I'm guessing abs) could have grown, or are retaining water due to being worked.
My waist measurement actually varies quite a lot day to day even when I haven't eaten and don't feel bloated. It's kind of odd.. I wouldn't worry about it too much if you're losing inches elsewhere, it'll even out eventually
Your body just decided it doesn't want to shrink your waist yet!