It is not your fault, it is called the "shifting screen". My DS had the exactly same thing, brought it back to the shop where I bought it from. Tell them that its an error happened when it was manufactured and that it started from inside. They can't see any physical damage that could have caused it.
Just to tell you more, it is one of the basic problems this device has along with the cracked hinges and bad d-pad. Everybody should check their touchscreens by very very gently pushing the screen with your knucle. If you hear a little click you should return it to nintendo once the touch screen stops working.