I am only doing 2 x 55 lbs dumbbell press (3 sets of 6 or 4 sets of 5). I don't have a spotter, so maybe that is why. It is pretty consistently my left arm. It is the only exercise that causes this. It comes and goes, last week I don't recall having an issue. Maybe my arms are tired from my other workouts? Maybe I should space them out more? Come to think of it, I was doing a lot of pushups every day leading up to this.