Benching Shoulder

Hi guys, i started training 1 year ago and now have some trouble doing bench press, overhead press in any variations, less problems when staying low weight but still. I also do 2:1 pulling: pressing cause i read it would benefit the shoulder.

When i bench i have some tightness/numbness in my shoulder and it´s not really painful but i can´t go on lifting all sets. I tested around myself and found that this tonygentilcore.com Bench T-Spine Extension Mobilization - YouTube exact stretch releases this nearly instantly. So right now i´m doing it between each set. But i would like to know the reason behind this. What happens in my shoulder? Anyone have a guess or had/has similar problems?