If I were to guess, you're a right handed golfer.
Where you've circled is muscular and I get soreness there when I bang a ton of balls as well. I think it's just a bit of overuse and it's gone after a day or so. In fact yesterday I played 18 and hit a bunch on the range afterwards and am sore in that exact spot right now. I'll hit a few more balls when I warm up but still plan to play today.