Apple watch suddenly measured too low

My apple watch suddenly tracked my workouts active calories far lower than it used to. This issue happened 2 days ago. Now the active calories for running on treadmills or strengthening just are 30%-40% as they were tracked before, although the duration time, the pace, the volume were all the same. I didn’t do anything with my watch, didnt change any settings. It just happened overnight. I unpaired it with my iphone, reset it, but the issue were still there. What should i do now?