What Causes Soreness And Pain In Armpit?

My 7 year old who weighs 50 lbs says that her left armpit has been sore for awhile. When I touch it, my child flinches and says that it hurts. She said that her other armpit was sore last year but is no longer. This morning she felt a bee like sting in her sore armpit. She does do gymnastics and softball. Could this be anything to do with her physical activities?