My question involves education law in the State of: Arizona

I am a part time university student going to a state college. My classes started about two weeks ago and the tuition was due on the first day of classes. The following week one of my classes increased the number of credits and now the bursars is saying that I owe them even more money since they increased the credits of one of my courses. To put into perspective they increased the number of credits of the course after the deadline to add/drop courses. This does not seem right by any means. Can they just increase the number of credits of my courses after tuition is due then demand more money?