Does she get them in any other part of her body? I remember that I used to get something that sounded very like this, although I never went to the doctor. It would be like a needle had just been pushed into my heart, and I would stop dead in my tracks, scared to breathe anything other than the most shallowest of breaths because any movement prompted the pain again. It would probably happen maybe once every couple of months, and then after a couple of minutes it would be gone, but I haven't had one in years.
The reason I ask if she had them anywhere else, is that I would occasionally get that feeling in other parts of my body, like my stomach or kidneys, and it seemed to be for no reason. I would just roll over and it would strike.
I would get a second opinion from a doctor, but I always presumed that it was just some sort of nervous system misfire in my body, as they came and went so fast.