We can make all kinds of hypothetical rhetoric on why it is or isn't the right size for you, but until you actually go out and start having aches and pains or find that it seems to limit your performance then none of this will really matter.
I'm 5' 11" and rode a 64 cm bike for over 35 years and was comfortable on it for the riding I was doing at the time. As I've started riding longer, further and faster I've sized down with several sizes between to currently a more sensible 56 cm frame and wonder if 54 cm in this particular geometry might have been even better.
3) neck muscles might just be getting use to having to do more work than previously. Or you are straining to keep your head up to see further down the road. In that case I usually find the helmet at fault. If you can't adjust it to stay out of the way of your vision so you don't have to raise your head up so far, then get another that doesn't obscure your vision when leaned over.
Last edited by Iride01; 09-17-21 at 08:43 AM.