I slipped.

No, actually, this is a good thing.

Slips are where you intentionally decrease lift on the plane, without nosing down (which would cause you to accelerate). To do this, you use the rudder to change the angle your plane is facing, then bank the opposite direction, to maintain your original heading. You'll maintain your airspeed - but with a lot less lift. As such, you'll sink.

What are slips used for?

• Allows you to descend at a steeper angle without using flaps, and without increasing airspeed.
• Some planes don't have flaps. Citabrias, for example. If you fly a citabria, you always slip to descend steeper.
• Maybe your airplane has flaps - but your electrical system parachuted out at 3000 feet, and left you to operate the plane by yourself. No power? No flaps!

Now, as to why you'd *want* to slip, instead of use flaps? It does provide an alternative to full flaps, and you can "undo" them once you descend to the level you want. You don't want to unextend flaps when you are close to the ground. Other than that, best I can tell, it is mostly a matter of practicing in case of an emergency.