What Causes Pain In The Armpit?

Our six year old son,soon to be seven, started crying with pain in his right armpit today. He cried for a few minutes ,would not let me touch him. He said it was just at the place entering the armpit, where the arm folds. It was gone within a couple of minutes , he could lift and move his arm as normal ,and there was no swelling. He was as normal after, we went out and he was running around playing etc. he had no temperature. When he had the pain he was in a lot of pain! He has recently started Taekwan - Do and is doing a lot of supervised stretching. He was swimming this morning with his dad, . Would this have anything to do with it? Just concerned as the pain seemed so bad