The below picture is to try to get you to imagine the arm pit as a cave. The top of the cave is the pec minor. The floor entrance is the lat/teres major, further caudal is the serratus anterior. Going deeper into the cave is the subscapularis. The wall towards the head is the arm muscles. The wall towards the waist are the ribs.
There should be a pocket deep in the arm pit. When there isn't, often times the muscles are dysfunctional that can create muscle imbalance or compression on the neuromuscular sleeve. As you palpate the arm pit, if you move the arm overhead, if you feel the roof collapsing very tightly on your finger/thumb that is palpating, often times the pec minor is very tight. If the floor pops up at you, the subscapularis or lat (depending how deep you are) is to tight/short. If the superior wall seems to cave in, the arm muscles (long head of tricep or coracobrachialis) is to tight.
Next time you are dealing with some shoulder/arm issues, don't get scared away from exploring the arm pit.