HRM question

MichelleWagner50

I changed my current weight on my hrm last night. When I worked out today, it said my heart rate was over 100 for my whole workout & that I burned over 300 cals. I did the same workout yesterday @ my old weight & burned only 175 cals with a heart rate of around 85. Is my battery going or would adjusting my weight by 5lbs make that much difference? Any advice would be helpful! PS. I have a polar f6 with the chest strap.