Go to the GP chap.
A day after playing 5 a-side i had excruciating pain in my groin,i thought it was a simple grion strain,when to the gp she disclosed that i had a hernia,even though there was no visible lump.
I was sent to see a specialist he however said i had stretched the tendon right up inside my groin which can be more painful than a hernia or a simple strain.
There was nothing that they could do but just to leave it to heal which could take upto 6 months. i was signed off for about 6 weeks in total,nothing i could do to relieve the pain.
At least if you get it diagnosed you may get some proper treatment for the nipper which maybe beneficial to someone so young.