I measure because my scale doesn't like to budge my my tape measurer does.
There's a lot of things to consider here:
1. Did you measure at the same time of day? I gain inches by the end of the day (sometimes 1-2" on the waist).
2. Did you eat right before measuring? That can also expand your stomach?
3. Is it close to your TOM? Again something else that will cause bloat...
4. Are you sure you're measuring the same spot? For my waist I measure 4 spots for comparison: under bust (so the band size of your bra), narrowest part of waist (typically 1-2" above your belly button), at my belly buttons, and widest part of my natural muffin top. ALL of those measurements are quite a bit different and lose in different ways.
5. Are you doing a lot of sit-ups/crunches? possible even with weights? Especially if you did this right after a workout it could be that you've built some muscle here and are also retaining water here too (don't worry, you probably didn't build 3" worth! but you could certainly be retaining that much water after workouts I gain up to 3" even when I'm not doing abs specific workouts).