When my PMR was active, I had pain in both shoulder areas and not just one side but you are right the PMR can come on quickly. My second occurrence did come almost overnight but the first time my PMR gradually came over several weeks along with gout in my right foot. I'm not sure a dermatologist knows much about PMR. I would try to get an appointment with a rheumatologist if possible. If you want to know more about PMR, there are some good videos in a discussion here:
-- Educational Conferences for PMR/GCA?: https://connect.mayoclinic.org/discussion/educational-conference/