I get this way too. A beer will taste amazing one day, then "meh" the next day. I think a lot of it has to do with mood, but also what you've eaten. I like to eat spicy food...a lot, so it really f*cks with the palate, even hours later, I believe. But, yeah, palates are bipolar like that. Sometimes, I'll perceive a beer differently even halfway through a pint. The first sip or two is always the best.