I don't know much about the tenant/landlord law, but if he saw that you had cats last year and said nothing and it wasn't mentioned at all in the lease, "No Animals", then I don't see how he can all of a sudden, turn around and change the lease for this year. BUT, if it was stated in your lease "No Animals allowed", then unfortunately you have no case. What do your parents say about this?

Good luck and I wish you all the best.