What Causes Pain In Groin Area While Exercising?

I retired last year December 2013. For the past few years I was employed as a proofreader, where I was sitting for most of the day.
Since I retired I have been very active, cycling, walking the dogs twice a day, and going to the gym.
I sometimes have a slight ache in the right groin, like toothache, which triggers down the back of my leg.
This does not last long.
Kind regards, Dave