I've started getting this recently, from google I've found it's a very common problem. There can be many causes, such as urinary and yeast infections, STIs, a pH imbalance, vaginal erosion (from friction) or vaginismus like someone mentioned. For some people it clears itself up quickly but for other it can be a long term problem that they are unable to find a solution to.
There are quite a few things you can try though, like creams available from he doctor, antibiotics, and some people say yoghurt helps - either eating more of it or inserting it - as it's supposed to restore the bacterial and pH balance.