By no means am I even close to an expert but my daughter is also learning IR and I found when she is throwing left she is closing her body (even just slightly) to the catcher instead of remaining in the open (45) position. I may have no idea what I am talking about but when she focuses on staying open it helps.
This is most certainly something to check.
My DD has been taking weekly lessons for a year and still sometimes closes too soon and that yanks the arm across her body, leading to pitching going outside (to a right-handed batter).
Stay open through release.