Is there anything worse than texting someone, only to receive a "k" in response? What could it mean? Are they mad at you? Are you no longer friends? Do they wish you were dead? Maybe you're a bad person?
Receiving a "k" means that things are anything but OK.
Besides receiving one-letter messages, texting in general is a dangerous thing to do. It can cause great physical harm, and not just to your thumbs.