This happens to me whenever I start an anti-depressant, or even while tapering off. Love the feeling but as it has been said, watch out for hypomania. My upbeat, energetic burst seems to last a few weeks then things calm down. AD's have caused hypomania for me in the past though, then I fell into a big depression so this is why I am on such a low dose of Prozac. (10mg instead of the usual 40-60mg a day)