Suggest Remedy For Lump In Arm Pit

I just woke up and ran my left arm past my right arm pit and felt this distention...thought it was a "sleep crease," but soon realized that something was sticking out of my arm pit. No pain, and it is only visible and touchable when I raise my arm above shoulder level. The distention became apparent 3 days ago; I stopped upper body exercise and the "swelling" has slowly gone down, but not away. Should I be putting ice on it to help the swelling, or just leave it alone?