Polar FT7 suddenly tracking differently after battery change?

I recently had to change the batteries in my FT7. Ever since I changed them, it seems to be tracking my calorie burn differently. Since the change, my hear rate appears to be about 10 bpm less than it did prior to the change at similar levels of exertion, and my calorie burn is about 100 cals less than it used to be for a same workout. The user settings are the same as they were pre battery change. Is it possible that, as the battery was dying, my heart rate and calorie burn were inflated, and now that it has fresh batteries it is only now giving an accurate read out? If not, what could be going on? Time for a new unit? Thanks for any insight.