When changing my baby's diaper I found a lump in his groin. What could this be?

Hard to say. A lump in the groin could be an enlarged inguinal lymph node, which can occur after a diaper rash or healing sore of the legs. Occassionally a testicle will retract into the inguinal canal and can be felt there. Inguinal hernias can also present this way. Best to have it checked to be certain. .
It could be... ...A lymph node or a hernia, and i would not wait too long before having your dr see the child.
Hernia or Lymph Node. A lump in an infants groin could be a sign of a hernia and should be examined immediately. If a hernia is found, surgery is required to prevent the associated risks of incarceration which is the trapping of bowel or other tissue in the hernia and is a medical emergency. Another possible "lump" is a lymph node which also may require treatment if due to infection.
A lymph node. Most often when parents find a small lump in the groin of a child, the lump will represent a lymph node. Lymph nodes essentially are the bodies trash cans and can get bigger with a recent illness. Groin nodes will respond especially to diarrhea type illnesses. However, lumps here can also represent other issues such as a hernia and should be looked at be your doctor to be certain.