Don't panic.
There's this weird thing with tests--we feel like we're not allowed to have off days, or that every day has to be our best score, because, darn it, I've been studying.
But an actor doesn't go out every time and give their best performance because, darn it, they've been rehearsing. An athlete doesn't go out and have their best game every time because, darn it, they've been practicing.
But for some reason we think that with something cerebral, like taking a test, that once we've reached a peak, we can only ever live at that peak. It's nonsense. There are good days and bad days, good performances and bad performances, days thinking is clearer and days where it's a little muddier (who knows why. Fatigue? Diet? Mood? All sorts of things!)
So don't panic. Figure out why your score was lower and what you can learn from it, but there's no reason to think that because THIS MOST RECENT SCORE was lower that your next test will be lower. Every time you take a test you have a range of possible scores. Over time, you want that range of possible scores to increase--you can reach a new maximum, and your minimum is higher than it used to be. But that trend is virtually impossible to guess from any single test. It's something you can only notice over time. All this most recent test told you is that a 640 is a possible score for you to get. Maybe it's the lowest possible score, and it's very unlikely. You've also demonstrated that you can get a 710. Great. Maybe that's your maximum, or maybe there's a score that's higher, who knows.
Point is, don't let a single test freak you out. Learn what you can from it, and keep moving forward.