Negative calorie burn recorded by unused fitbit????

I have been on vacation the last week and did not take my fitbit--however, i am now noticing that my fitbit is taking exercise calories away from My Fitness Pal daily goal. Today it registered a -165 calories subtracted from my exercise calories burned and i am not even using it today. One day it recorded a -465 calories. Why is it doing this all of the sudden and how do i make it stop?