I think you're experiencing a combination of effects.
1). Palate Fatigue is very real. Kind of like when you cook something with onions. You stop realizing your house smells like onions until you go outside and come back in.
2). I recently read, either in Mitch Steele's IPA book or the Brewing Elements Hops book, that your body chemistry can actually change the chemistry in the glass as you drink it. Saliva + Hoppy chemicals = differing tastes from top to bottom of glass. (Maybe that was a dream I had, though...)
3). You suffer from homebrewer self doubt! I have the same affliction. My opinion on a batch can change from glass to glass (or sip to sip!). That's when I have to remember that I love beer and to not worry about it!