LOTS of things can change your measurements (even just temporarily).
All these things affect your measurements:
-TOM/ovulation
-Heavy lifting (any workout for that matter) but heavy lifting tears your muscles and they retain water for a day or two afterwards while your body repairs the muscles
-Too much salt
-Did you measure first thing in the morning because I get wider as the day goes by!

-Heat
Also, just basics, make sure the measuring tape is parallel to the floor (hard to do sometimes!) and that you're measuring from the exact same spot. Don't even bother comparing measurements in different bras because those will be different for sure.