if you hit your sciatic nerve the your not injecting in the right spot , as it runs very close to the crack of your ass.
their are alot of nerves all over you body , you probably just got one of those. I would go see a doc though if the problem presist.
it could also be from injecting to much into one spot , break you shot in half and take say 1 1/2 cc in each glute or thigh.