First off, it will be your rotator cuff, even though that may not hurt. Its pain transfer. Get some myotherapy done on your shoulder. Do not train any upper body for at least 3 days after. Give it 2 weeks and if its still bothering you, get another session done. If the injury has been there a while it may take 2 or 3 sessions to come right.
Also, do rotator cuff rehab work. Do a search on youtube. Shoulder dislocates with a broom handle are good.
Secondly, military press (the one with locked knees, not push press) should be done with feet no more than shoulder width apart, a little less is even better. Think, "pillar of power". Hands should be close in, around the beginning of the knurl. I'm 189cm tall and around 120kg so my grip is about 2cm out from the start of the knurl. A short smaller guy may be gripping inside the knurl a bit. The further out you go, the less power you will have, even though you will get a shorter bar travel path.
Have the bar in a squat rack so its around the height of your upper chest. In other words, almost zero lift needed to unrack. Take your grip and move into the bar. With your narrow grip your lats should be bunched up and tight. They will form a powerful lift off platform to get the bar moving. With a wide grip, you never get to use your lats, making you effectively weaker.
Squeeze your glutes tight to fire them up. Crush the bar with your grip. Push air into your gut to firm up your core. A lifting belt will do wonders here. Also, while you are at it, use oly lifting shoes or something with a thin hard sole.
Unrack and step back a touch. Your lats should be bunched, your knees locked and your core tight. Chances are that if you are dong it right the bar will not be on your chest because your lats will be in the way of your arms.
Do your shit.
Rerack and wait for the applause.
Chit yeah brah.
I'm pretty sure that there are some setup vids by Rippatoe around.