The Bump says the term "lightning crotch" refers to sudden intense pains that occur in the crotch and lower abdomen during pregnancy. The Bump attributes these pains to either the baby repositioning itself or to early Braxton Hicks contractions. WebMD, however, attributes pain with these characteristics to round ligament pain.
WebMD says that during pregnancy the ligaments that support the uterus get stretched. Pain from the stretching tends to occur suddenly, lasting only a few seconds or minutes. Often the pain is felt in the crotch area. Usually moving, such as standing up or rolling over, will trigger the sudden sharp pain of round ligaments stretching.