How to age 4 years in 4 weeks

I have no idea why my Garmin thinks that I've aged 4 years in the last 4 weeks. I went from 35 years to 39 years. Didn't make any changes to my workout routine, my sleeping schedule, my diet. I didn't have any abnormal amount of stress. No crazy changes in heart rate either.

I've been wearing my Garmin for 8 months and this is the first time it's acting this way.

What could be the explanation?