We have a four year old female shih tzu who has not been speyed.
She is always licking herself (down below), obviously dogs do lick themselves but she licks herself more than any other dog we've had. What could be the reason?

There could be a number of reasons and I would say that the best course of action is to take her to the vets and have them check her out. It would be very hard to guess on here. :)