Hello
@baokhiem!
The way your puzzle rating changes after each puzzle is just as if you were facing an opponent with that puzzle's rating, which means you earn more points when you succeed in a hard puzzle than you would with an easy one, and likewise after a mistake you'll lose less points against a hard puzzle than the amount you'd lose with an easy one.
In this particular situation you probably just got a few mistakes on easy puzzles and it happened just like those unlucky blitz arenas where we face a few underrated dudes and lose for silly reasons, so I suggest you keep up the good work and you'll see your rating increasing again in no time (specially after a sound sleep because our brains need it) :D
Have fun!