That’s how I noticed the problem first, and thought it was due to the exercise.
Could be something else though. At least if it hurts a lot, you can try to move the watch to the other hand to see if it stops (or if unfortunately the other hand starts to hurt).
I think the only useful information for VO2max is when you open the yearly graph and then you can see the trends. Otherwise, day-to-day, not accurate. Needs maybe a month of training to spit out a remotely useful number.
But speaking of this - VO2max of around 40 sounds about right, if you’re not super active, running a few months. It’s not really an indication of durability, how long you’ve run. The estimate is more about how much energy you can spend for a given heart rate.
In a year or so, if add some sprints, HIIT, hills, you’ll be 50ish and rising :)
Its series 7, 45mm.