What Causes Pain In Inner Thigh And Groin Area?

I'm 23, 5'3" at 135lbs. I've never broken anything or had any major surgeries. I've done alot of hiking in my life but recently as I've been hiking I experience a sharp pain on my inner thigh/groin area that makes hiking up hill extremely painful and after a while its almost impossible to raise my leg. The last incident was about 3 weeks ago and the pain started after about 3 miles. Then yesterday I went hiking and it began after only 1 mile. Whats causing it and how do I get it fixed?