Loose front rotor

I've been hearing some occasional noise from the front right and this
weekend took some time to figure out what it is. After taking off the
front right tire, at first I noticed that the 4 wheel posts were loose
relative to the rotor, but after trying to move the rotor, the rotor
itself wiggles back and forth a half inch or so (which I'm assuming is
BAD). I took the dust cap off the wheel hub nut (hope I'm calling the
right thing) and the nut itself seems very tight. What else might
account for a loose rotor?

The only work I've done is to change the front brake pads several
months ago (although the left rotor assembly seems very tightly
attached (no movement)). Would any work of moving the brake calipers
around have affected the rotor looseness?

Other than the wheel hub nut, I'm not sure what to look at. Any
suggestions?

Thanks.