Although there can be various causes for a lump in groin region but most important ones in present context seem to be
in the leg
Infection in the leg
Sexually transmitted disease
like chlamydia or gonorrhea
If you have a history of STD
then you require thorough investigations for the diagnosis
However if there is no history of any sexual contact and there is some infection or injury in the leg,then a course of antibiotic should make the problem alright.
Consult your doctor for the treatment.