What Causes Severe Pain In Groin During Pregnancy?

Hi. I am 31 weeks pregnant in two days and for the past week I have been having serious discomfort and pain in my groin area (folds of top of the legs and vagina) it s becoming increasingly hard to walk. It causes me severe pain when turning over in bed, opening my legs and standing on one leg, when putting trousers on for example. What is this? Because it is starting to really get me down and worry me. Thank you in advance.