Without examination, it is difficult to say for sure. A groin strain is the most likely cause, but other things are possible - including a hernia or inflammed lymph nodes. If you had a recent GU infection, lymph nodes are more likely.
You can consider imaging the area (i.e. with an ultrasound or CT scan) to determine if there is a hernia. A referral to a surgeon can be considered for further examination.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
Medical Weblog:
kevinmd_b