All the courses I was taking in senior year were extra courses. I had already fulfilled my required credits.
Whenever a student makes ANY sort of academic change after applying to college (even if it’s something minor– like switching from a cooking class into woodshop–and even if all graduation or college-entrance requirements have been completed), it’s imperative that colleges be informed right away.
Your change of plans is, of course, is significant, and so the college that admitted you should be notified right away. You need to explain exactly why you made the switch to the new school and how the differences in school calendar disrupted your schedule and caused you to lose a quarter’s worth of credit.
It is HIGHLY UNLIKELY that this will have any negative impact on your acceptance. I certainly don’t think that admission officials would rescind your acceptance based on this change UNLESS the classes you are now taking at your new school are much less demanding than those that you were taking at your old school. But, in any case, you must notify your college today. Likewise, if you have any other college decisions pending, you must notify those schools as well.
Again, assuming that the rigor of your current program is at least roughly equivalent to what you left behind, you should be fine … as long as you clearly explain your situation to admission officials immediately.