Today I got a new vo2max value (the upper one in the picture used to be 47) although I haven't done any activity for days since I'm sick... How come?

You can get new VO2 max values without doing activities.
https://support.garmin.com/en-US/?faq=lWqSVlq3w76z5WoihLy5f8
"VO2 Max Estimate From All-Day Heart Rate
Newer watches are capable of generating VO2 max from all-day heart rate data. VO2 max generated from all-day heart rate will only show if you have not generated VO2 max from another source above, unless the all-day heart rate VO2 max is better than VO2 max generated from other sources."