Yep, I get those 'up' days too, usually related to domestic contentment ie never when I am doing something amazing, but something mundane like serving an edible looking meal (admittedly a rarity). I feel like I could conqueror the world and want to run around making daisy chains (not at all 'me').
Then a day or so later, I'll be wondering where the hell that person went. She's suddenly been replaced with a strange woman secretly convinced she's about to have a heart attack who sees the pleasure in nothing.
I always assume it's when my oestrogen is high that I feel so good (ie, the 'normal' , pre hormonal chaos, ovulation feeling). The next day must be the sudden dip afterwards?
Either way, the only good thing is that neither lasts long!