What Causes Severe Pain In Armpit When I Move Arms?

hello, i am dustin monette. i injured my shoulder and have recently began mobility exercises. the problem is that on the inside of my armpit when moving my arm laterally greater than 90 degrees i feel a sharp stinging pain on the surface of my skin almost like its tearing, its also very red in the area of the pain. this is slowing down my mobility rehab and i would love to know how to treat it