Apple Watch Battery Going Down While Turned Off

Iv had my Apple Watch since December. It’s series 5 cellular. I always turn my watch off overnight and turn it back on in the morning for use. I found that by doing this I could get 2-3 days battery from it, depending how long in the day I used it. On Friday I turned it off at 73%, I didn’t turn it back on all weekend and on Monday morning when I turned it on it was only on 28%. Throughout the day it stayed on 28% for around 12 hours before it started to decrease. I charged it Monday night overnight and Tuesday morning it was on 100%, within minutes it jumped down to 83% after just taking it off charge. During the day the battery was fine and didn’t drain fast or anything. I turned it off overnight again at 73%. I have turned it on this morning and it is now on 50%. I am really confused as to why this is happening, it never used to decrease the battery when it was turned off, I don’t understand why it should. When using the watch the battery doesn’t drain fast it’s still normal during use. Has anybody had this problem and can help??