The weight of your baby's head is compressing the nerve endings in the bowl of the pelvis, particularly the nerves that are associated with the spinal cord.
Also, within the boney parts of the pelvis is something called ischial spine, and it isn't a spine, but a bony protruberance within the pelvic opening.
You may sometimes feel a similar (lightening) feeling when your doctor does a vaginal exam, because the bony protruberance is noticable by the person who does the exam. If the exam is rough and fast, it is more likely that you will feel these "lightening strikes". Midwives are taught to avoid this area during the pelvis exam, because of the obvious discomfort.
Imagine being the one to do the pelvis exam. When you slide your fingers into the vaginal opening, if your sweep your fingers from side to side, you can feel the bumpy ischial spines on the back of the pelvis on either side. Search wikapedia for a "map" of the pelvis and you will see the ischial spines and just where they are located.
Try getting on your hands and knees to get some relief from the feeling.
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