NSFW: Why does doggy-style hurt my g/f so much?

My g/f dislikes having doggy style sex, because it causes her immense pain. As she described it, it feels like having a knife inside her. This problem is only there for doggy-style, and we have no issues with other positions.

I am somewhat larger than average, but not so large that I would think it would cause this type of pain. I’ve never had this complaint (at least told to me) by any of my former partners (and there are over a dozen). My g/f also seems to be new to this type of pain with this position (though I’ve not really been keen on asking her about her past sexual adventures to find out the the specifics).

Does anyone have an idea what could be causing this and/or how to prevent it? I really enjoy doggy style, but it’s a problem if it’s just excruciating for her.