Bigger heavy bag gloves causing pain to my Rotator cuff?

I don't know if this makes sense, but whenever I use my heavy.. heavy bag gloves and hit the bag for 15-20 + minutes it tends to cause some pain and instability in my shoulders, particularly my RC.. but when I use smaller heavy-bag gloves I don't feel any pain at all in my RC.. is there any reason why? I've had some silly reasons why, like more '' shock-waves '' throughout my arms, but i'm sure if it's not all in my head you guys would know.

Just to add some info I had my shoulder pop out then sublux about 4-5 times so I couldn't even hit the bag with the arm for 3-4 months then could only jab for another 2. I just started to use my left-hook this January with full power - so it's not like my arm was still really bad - I was hitting just as hard with it as I use to, so i'm not sure why this happened.