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The most common cause of this in an inguinal hernia. They are hereditary but are easily repaired.
The danger comes in because the intestines push through the abdominal wall at the tear and that is the lump that you are seeing.
You need to see your vet on Monday for an exam and to schedule possible surgery. You can usually plan a neuter for the same time and save some money. Dogs with hernias should not be bred.
Let me know if you still have questions.