What Causes Soreness And Cracked Lip In The Vagina After The Intercourse?

Hi, me and my girlfriend, had our first time around 3 weeks ago and it went very well, only a little pain to begin with. We tried for a second time last night, this time she was in much more pain and we didn't actually manage to have intercourse. I had a look, and just below the vaginal opening, it looked very sore, almost like a cracked lip would look, and it was sore to the touch. We are certain it is not an STD, but would like to know what it could be? what to do? and how to prevent it happening again?