The two concerns that first come to mind are testicular torsion and orchitis. The first involves a twisting of a testicle from its normal position and is a medical emergency that requires evaluation as soon as possible. The other is an inflammation of the testicles (e.g., by bacteria or viruses). It is unlikely that this is bilateral testicular torsion, but it would be wise to be evaluated today for either concern.
Testicular torsion reference: http://www.mayoclinic.com/health/testicular-torsion/DS01039
Orchitis reference: http://www.mayoclinic.com/health/orchitis/DS00602