A few days ago I got an email form a concerned Dad. His name Jeff, and his son is having low back pain on the landing side when he pitches. We spend a lot of time studying and talking about arm pain, but this is something I've seen seen enough that I though it might be worth sharing.
Here is my response to Jeff's question.
I can't say that I see this frequently, but it does come up from time to time, and when it does, it usually appears on the left side for right handed...