The Ready State Virtual Mobility Coach is like having a virtual Kelly Starrett in your pocket.
I’d start by looking at whether you are able to fully externally rotate your arm. A lot of times your dominate side does a lions share of the work in these movements and if you aren’t able to fully stabilize your shoulder, you’ll end up with pain. Here is a good vid to start with:
Followed by parts 1&2 of this vid: