I don't think she has a right to be hurt. You told her BEFORE she picked the day that you would NOT be able to come if it was on that one day. You didn't tell her to change her wedding day. You told her before she picked her date that these were the parameters for your attendance. That's not unreasonable.
Maybe this is too much of an assumption, but I've known a lot of people who will set you up for some kind of "friendship test" to see if you measure up to their ideas of what a "true friend" is. Usually they're controlling, demanding people. If she fits that description, it may have been a test. Based on the communication blanketing, I think she's trying to bully you.