Maybe it's because you made such a big production of it, built up hopes for it, had great expectations for it. And even though it was a success it wasn't mind blowing, it didn't make the earth move and you're still Killerasp and she's still just some girl.
It think you're just feeling a little underwhelmed - subconsciously thinking "I did all that work, suffered all that nervous energy, and now it's over and I have to do it all again?"
You've had your first date now, but just remember to make sure that dates 2 thru 100 are more relaxed, more fun, and have less expectation heaped upon the outcome. Your attitude going into a date can often have more effect on your feelings afterwards than what actually happens during the date.