Relax, regularly do flapping wings and shoulder circles while riding to identify when/where you are too tight.
But it's muscular. You will adapt and develop. Otherwise, go see a physio to do so.
I can't imagine the V very upright posture and handlebars being the root cause though.
You might simply be holding more weight with the back, now that you don't rest on handlebars. So, straight back is important.