Fenix 5s skiing - suddenly watch shows completely wrong denivelation / descent and distance I skied.

Hi, 
I usually (3 years) downhill ski the same slope . Official denivelation is 450m, watches was showing always  around 460 - 480 m each ride and I was ok with this spread . But today it showed completely stupid descent numbers. 600m , 26 m,  480m, 40m, 900m .....  the same difference occurred for distance I skied... 

Would you advice what to do please ?  i have not changed anything since Saturday ( when i skied last time) and it has worked properly... 

Thank you a lot 
Kind Regards
Pavel