left armpit pain

Woke up today with pain under the left armpit but it wasn't too bad. I looked for lumps and to see if it was swollen but it seemed normal, no lumps, not swollen, and just like my right armpit, which is perfectly fine. the pain went away and i forgot about it until around 6:00 pm when it came back. the pain  isn't too bad its just annoying. the pain just comes and goes randomly. the pain is just inside the skin near where the muscle and skin touch (that where i think it is.) I'm a healthy male, 16 year old.