It is unlikely that a swollen lymph node in the groin is being caused by a ankle strain. Typically lymph nodes get enlarged when there is a nearby infection in this case a groin or a urinary tract infection which may be draining into the lymph node and thus leading to the swelling. I would advice you to keep a watch on it for a few days and also analyse whether you are having any other concomittant infection in the area which may need to be treated. If the swelling does not decrease then please see your GP in person for a more detailed examination and a possible biopsy if deemed fit which at this point i feel is unecessary. All the best
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