A whiskey can taste different each time you taste it for reasons that have nothing to do with the whiskey or with it changing in the bottle. Whiskey changes very little if at all in the bottle and usually noticably only under extreme circumstances, such as when a very small amount of whiskey is left in a bottle for years.
The more likely explantion is a change in something else such as, not to put this indelicately, whatever you had in your mouth right before you drank the whiskey, or some different aroma in the air, even your own body odor can affect how your drink tastes.
I'm not saying you stink or that whisky can't change in the bottle, just that a change in something other than the whiskey is the more likely explanation for your varying taste experiences.
As I have commented on this myth before I'm happy to see someone with a little cachet help continue to slay it. Damn I'd make it a sticky. For me, recently cleaned wood floors (especially Murphy's oil soap) are a notorious taste changer.