Some common causes:
Enlarged lymph glands in the groin area
Hernia (usually a soft, large bulge in the groin on one or both sides)
Injury trauma to the groin area
Sexually transmitted disease
Since you have been tested for STD and UTI, there are other tests that may be done:
Blood tests such as a CBC, blood tests to check for syphilis, HIV, or other sexually transmitted diseases ,Kidney function tests.
I think that groin lump could be caused by an inguinal hernia. You could begin taking some anti-inflammatories, like ibuprofen for 2-3 days, but if the lump persists, you should go to your doctor.
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