Back fatigue...stem length/height?

I’ve noticed that as I ride my (road) bike more and more that I’m developing general back fatigue and it gets worse the longer I go. I belive my stem is too long (I think it’s a 130/140mm), and that it’s parallel to the ground rather than angled upward. Would changing this angle and/or legth help this? I’m rather new to cycling but I thought this would be the best place to turn.

We need a picture to be of even the slightest help.