Muscles burning? Obviously you fubared the aerodynamics of your S5, gunna have to trash it and start over

have you tried lowering it? sounds like it's a little too high. But I am far from anyone who should be giving fit advice...

Mine was a bit (and I stress a bit) to high and gave me some lower back issues. I would also get a little knee pain upon the bottom apex of the pedal stroke.

Is your seat straight? having it off center may twerk your back a little as well.

YouTube how to do your seat post fit to get a general ballpark and go from their is my suggestion.