It could be that your playing has worked a few germs into the boundery between the edge of the nail and the skin. This is a fairly comman occurance for me. It happens two or three times a year.
I'd suggest carefully squeezing along the nail line to see if a bit of pus comes out, then working some rubbing alcohol into the side of the nail.
If it doesn't clear up in a couple of days, or gets worse see your doctor.