What Can Be The Cause Of Pain In My Shoulder Near Armpit?

Hello.

Last week I woke up with a pain in my right shoulder. It's not on the front of the shoulder but behind, right on top of the armpit - it's quite difficult to tell exactly where. It started off as a dull ache, but I woke up a few nights ago with an intense ache in the same area, it was agony. I have been taking ibuprofen in the day and it has been helping with the pain, but whenever I wake up in the morning it's back to the same agonising pain. Aside from the intense aching, it feels like my shoulder has physically popped out of place - that's the only way I can describe the feeling.

I have also noticed that when I stretch my neck to the left, I get a a sharp shooting pain in the affected shoulder.

I am positive that it's nothing to do with the joint as I'm able to move my arm around freely without pain. Although, when the pain's at it's worst, usually in the morning, I find it difficult to move the arm up and down slowly. Quite painless, but difficult - as if the arm doesn't want to move.

I should note that I weight-lift heavy regularly, although I did not feel any pain in the shoulder whatsoever before the incident.

I am 19 years old, around 6ft in height and around 200lbs in weight. I have no medical history.

Thank you.