This is something I have experienced as well, I have been a user of Apple Watch since day one. I can also get it from the Sport Band.
What I can tell you, if you want to give it another try before seeing a doctor is “clean your watch”, the Series you’re using a waterproof version.
Did you exercise with it? Or got any sweatier lately? If so, this is what happens to me, after sweating and exercising this is more common. What I do now is, after each workout, rinse my watch under the tap, use a towel to dry it out and then use the feature to remove any water left in microphones and speakers.
Let us know how you’re doing, as this could be helpful for new users.