I have the same issue. It only happens when I am walking and stops me in my tracks. It feels like your getting stabbed in the vaginal area and only last a few seconds, right? I googled it and found out that as the tissues around your pelvic bones thin out and baby puts pressure on the pelivc area it causes sharp lightning like pains. This is completely normal. I started getting them around 27 weeks I am 34 weeks now and still get them especially when I wear prego jeans...
I experienced this yesterday three time in a row and it got my attention real quick. Apparantly this feeling has a name and it's called lightning crotch LoL :)