It is not easy for many people who are not familiar with the Snapchat text slangs to understand short forms such as MK.
This person is mad at you but not willing to talk about why.
However, unlike in chatrooms, in gaming, AFK is more commonly used to describe players who are inactive or not currently playing.
We're going to run a 5 K this weekend.
It is also used to make fun of an individual by sending them something interesting by typing "ICYMI" in it.
Are u mad at me? While sending this slang, a person is expressing that they are going to tell you something that you might have missed out while chatting.