Apple Watch series 4 won’t hold a charge longer than 6 hours

Hi,

I got my series 4 watch last November and it’s been working fine until last week. All of the sudden it won’t hold a charge longer than about 6 hours. I charge it overnight, leave for work with it at 100% and by noon it’s completely dead whereas previously it would last all day and remain at about 40% when I was getting ready for bed at night. No location-tracking apps have been downloaded and usage hasn’t been increased. I use it mainly for workouts, regular notification (text/weather/news) checking. I’ve restarted it and unpaired & repaired with my phone with no luck. Any advice?