@Anna1031, did the host give a reason for the increased price?
(Whether they did or not, you should certainly contact AirBnB directly about this.)
And yes, it has happened to lots of other people.
Sometimes it is because the guest made a mistake during booking that the host is trying to correct (such as stating the wrong number of guests).
Sometimes it is because the host made a mistake about pricing.
Sometimes the host is just being greedy.
Contacting AirBnB will help sort it out.
AirBnB can also help you find alternative accomodation should your original host cancel on you.