Your initial upswing has run into the inevitable, and to be expected, downswing.
DON'T PANIC! (You know, like the hitchikers guide to the galaxy.)
This happens. If KK loses to AA, and QQ loses to KK, that is how it should be. It does happen, and will happen to you again in future, if you continue to play. You will win your share of hands also.
To be sure you are playing correctly, you should post some hands in the forums. Have them looked over by those more experienced. But, even if you are playing 100% correctly, you will suffer the swings that give, and then take, your money. Playing correctly doesn't guarantee that you will win the correctly played hands. You will be busted by bad play from your opponents. This is a good thing, though it may take you a while to feel that. If bad players couldn't win some money from you, you would never be able to collect your winnings from them.
Keep playing, keep reading, post hands, everyone here has experienced what you are describing. Keep making the correct decisions, and you will make money, over time.