I have this, too. It's caused by the hormone relaxin, which loosens the ligaments in the hip and groin area... Some women also suffer from recurring hip-dislocation because of this during pregnancy. If it gets really bad as you grow, your midwife may refer you to see a physiotherapist who can give you a 'femibrace', which will take the pressure of your growing bump away from your legs and back and hold you all in place.
Good luck, I hope you don't get the pain for too long!