Could there be more to it?

Hello people, me and my girlfiend have been going strong for almost 5 years now. I have had a relationship before, while this has been her only relationship...we have been steady and are considering marriage. Lately ....since 7/8 months both of us have been really busy and dont meet as often. She has to travel frequently on work purpose.

Few days back we had a fight (on silly thing) and we told each other nasty things. She....in a fit of anger said that "you dont even know how to kiss or f***". I confronted later how would you know if i was good or bad when you havent had the chance to compare the performance with (i know kiddish) with to which she said she was just getting back at me and I am the only one she has had a relationship or had been physical with.

But something tells me this is not as straight-forward and there is something more here....but dont know how should I go about now....

True or not, I'd have grave concerns about marrying someone who would say this to me. How often does she indulge in this type of behaviour?