May be she has an issue with her Uterus... If you are jsut doing foreplay and not intercourse may be she is more excited during the foreplay. try to see a doctor and check what might be the real cause.
The pain might be due to the Shape of the uterus also.
Usually pain during the initial stages of penetration is due to UTI and vaginal infection or vaginitis, a tight vaginal wall, sores at the mouth of vagina or vestibulitis, vulvodynia or a dry vagina (frictional injury), thrush or fungal infection. Pain on deep penetration is due to pelvic inflammatory disease, endometriosis, irritable bowel syndrome, pelvic tumors, cervical and uterine infections and tumors. You should get these possibilities evaluated from your gynecologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.