I understand your concern. The pea sized lump as you said could be a lymph node in the inguinal area enlarged. The cause is usually an infection in the drainage area like foot or anywhere below the gland.Infections could be a folliculitis or a pustule or simply a small abrasion that occurs while playing. In such cases I would look for the focus of infection and treat with an antibiotic cream. As there is pain I would give antiinflammatory analgesic like Brufen syrup and observe Once the infection is treated the lump will also go away in some time. Take care and inform me if any more doubts. Observe if the swelling increases while crying or coughing in which case it could be an inguinal hernia. If you have more worries please clarify with your doctor and have him examined.