I had numbness under my arm (no pain) after removal of lymph nodes - removing them means stretching tissue about quite a lot and can affect nerve endings. I nevertheless has full mobility of my arm, just a bit of a problem applying deodorant. 4 years later I still have a degree of loss of feeling (can't distinguish hot and cold much so good job I don't wax!) but otherwise normal. It could be scar tissue but may also be fluid. Very likely to improve as it's early days but no harm in checking with your GP.