Sharp, stabbing vaginal pain is common during late stages of pregnancy. It usually occurs as the cervix prepares for dilation. However, check with your doctor if the pain is sever.
The baby may be kicking and moving around on your cervix or your muscles are contracting since you are in the 36th week. Your insides may have to rearrange itself to make room for your baby so you'll feel pain due to this movement.
During the pregnancy period, not particularly in 36th week normally the mother's body required plenty of water. The shortage of water drinking may be cause sharp vaginal pain.
The sharp pains means that your baby is coming done and taking position to come to this world. It's time that now you be careful as you will deliver soon.