The route I was thinking down was of a Spotsman's Hernia (otherwise called Gilmore's Groin). It is a common injury in footballers and is normally causes a dull nagging pain deep in the groin.
If no other treatment for a groin strain is working, you could try some rehab for this condition to see if it helps. This involves core stability work. Best using a gym ball to tone up the deep lower abdominals. There are numerous exercises so might be best looking on the internet for some or buying a book.