I have this exact same problem that has only just appeared. I have been wearing an Apple Watch for over 5 years every single day, and just recently I have started to experience these small marks on my wrist underneath where I wear the watch. I am currently wearing a series 8, and have done since it was released, but it is only in the past month that I have experienced the issue. I wonder what is really going on.
And no, I don't need to see a doctor, and I would like to be able to continue to wear my apples watch, as it has literally helped change my life.
I will maybe try the option recommended of turning off the heart monitor, although that is quite a big reason why I wear the watch.
I do hope Apple are looking into this, as it seems I am not alone.