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Please clarify: Is this a firm, rubbery lump that is slightly moveable located along the crease between the abdomen and the thigh?
With presumably no trauma, you appear to be describing a reactive lymph node, a filtering gland of the immune system that swells in reaction to infection (e.g., a "cold"), among other things. Lymph nodes are usually not tender, but can become so when reacting and doing their job. The fact that yours is quite painful could indicate the need for physician evaluation for antibiotics, etc., if it involves any of the following: fever; skin changes (redness, swelling, bleed, clear or pus discharge); growth to more than 2cm (about an inch) diameter; marked pain, especially beyond 24-48 hours; persistence beyond 2-4 weeks.
Is there any personal or family history of lymphoma, leukemia, or tumor/cancer?
This is the same kind of lump that you might feel under the corner of your chin when you have a cold or sore throat. Being part of the immune system, lymph nodes can swell up even if the cause is elsewhere in the body (I've seen groin lymph nodes react to a nasal sinus infection).
Please follow the guidelines in the prior post about seeing a physician if any of the "red alert" criteria are met.