I suggest you fire it from a rest to determine whether or not it's something you are doing...grip, trigger jerk, flinch, etc. My M&P9c also was shooting right - both off-hand AND from a rest. Based on this, I determined that the rear sight needed to be drifted to the left. A few taps with a brass punch and it was good to go.
For what it's worth, I'm also a lefty. While this may not fix your problem, it worked for me.